If campaigners for greater public discussion of Britain's immigration policies are as serious as they claim to be they'll have to do better than putting the bumbling former Archbishop of Canterbury, Lord Carey, on the airwaves to promote their case – as they did today.

Personally, I'm in favour of such a debate, though well aware of why the major political parties run a mile from it. It's become the dog that almost never barks, a fact I find amazing.

Why so? It's a complex and emotive issue. Some voters who wouldn't dream of doing society's rotten jobs themselves want to "send them all back" yesterday. The BNP and its like hover malevolently without having anything useful to say either.

But here we go again. Last night the Balanced Migration group, co-chaired by backbenchers Frank Field (Labour) and Nick Soames (Conservative) issued another challenge ahead of the election. It should be on their website but wasn't there when I checked this morning. Among the papers I read at home it was reported only in the Daily Mail, though BBC Radio 4 carried a report – and interviewed the hapless-but-decent Dr Carey.

Anyway here it is:

A DECLARATION ON POPULATION

70 MILLION IS TOO MANY 1. We are gravely concerned about the rapid increase in the population of England that is now forecast. We note that the official projections show the population of the UK will increase from 61.4 million in 2008 to exceed 70 million by 2029. Over the next 25 years the population will increase by 10 million, nearly all of the increase being in England. 70% – 7 million – will be due to immigration. We believe that immigration on such a scale will have a significant impact on our public services, our quality of life and on the nature of our society. 2. We welcome the considerable benefits that immigration has brought to British life. However, we note that over the last decade immigration has reached unprecedented levels. Furthermore, we note that a major enquiry by the economic affairs committee of the House of Lords found no evidence that net immigration generates significant economic benefits for the existing UK population. We note also that the latest household projections show that nearly 40% of new households to be formed by 2031 will be the result of immigration – approximately one every five minutes. 3. Accordingly, we call on the major parties to make clear commitments in their general election manifestos to reduce net immigration to the levels of the early 1990s – that is less than 40,000 a year compared to 163,000 in 2008 – in such a way as to ensure that the population of the UK will not reach 70 million. 4. We recognise that this will not be easy to achieve. Over the last decade or so we have lost control of immigration. It will take several years to put this right. But the first requirement is a clear political decision to put in hand the measures required to restore control over our borders, to break the present almost automatic link between coming to Britain and later gaining citizenship, and thus take a range of further measures to limit the growth in our population. 5. We are convinced that failure to take action would be seriously damaging to the future harmony of our society. Nearly a million votes by our fellow citizens for an extremist party amount to a danger sign which must not be ignored. For too long the major political parties have failed to address these issues and the intense, if largely private, concern that they generate throughout our country. If politicians want to rebuild the public's trust in the political system, they cannot continue to ignore this issue which matters so much to so many people. The time has come for action.

Plenty there to take issue with, but also to agree with, I imagine. Even those who say that immigrants produce a net economic benefit to society usually offer figures that suggest only a modest net per capita gain. And the point about the strain on public services at the poorer end of society is hard to deny. My GPs are a Pole and a Sikh, both excellent in their different ways, but I don't live in a deprived neighbourhood and they don't choose to work in one.

When the 70 million projection first emerged from the Office of National Statistics the Guardian's Alan Travis raised some expert questions about the likelihood of it ever actually happening. So did the immigration minister, Phil Woollas, who is far from being a bleeding-heart liberal (that may be why he got the job), but said the government's tighter rules had already produced a 44% fall in net immigration in 2008.

"Net" is always an important word because it implies that some left too. And 2008 wasn't a great year for economic migrants – those from eastern Europe mostly – to head this way, what with the recession and the pound's sharp fall against the Polish zloty.

I doubt very much whether UK population will reach 70 million by 2029 or thereabouts because population projections usually tend to go wrong. We were at ZPG – zero population growth – in the mid-70s and some of the same people now alarmed about there being too many of us were alarmed about there being too few.

But these things should be talked about more than they are. It's not just raw numbers, it's who exactly the newcomers are, the contribution they make to the economy and wider society, the demands some may also make.

We are usually happy to accept highly skilled doctors, engineers, IT professionals (well, Nick Griffin might not be, but most people are, I expect), and are worried about too many poor and illiterate people (not to mention militant Islamists) who lack the ability – sometimes the will – to work.

Refugees are a separate category. It's right to take in the needy, as we have (to great benefit) for centuries; wrong to be deemed a soft – or administratively incompetent – touch in a world of globalised travel. Labour finally seems to be deporting more people who shouldn't be here or should have left. Good. That may restore some battered confidence in the system.

The signatories of today's Balanced Migration statement are, at best, second order politicians. Field is a clever chap, but a hopeless practical politician. Soames is a beached Tory whale who carries the curse of Churchillian genes. Carey was Margaret Thatcher's choice for Canterbury who looks smart only in comparison with the bearded egghead, Dr Rowan Williams, who succeeded him.

Betty Boothroyd is there, along with Michael Ancram, Peter Bottomley and Peter Kilfoyle – a ragbag of decent, essentially liberal folk without much clout.

On Radio 4's Today programme this morning, Carey explained that he isn't asking for only Christian immigrants – glad we cleared that up – but that newcomers must accept the rules of our Christian-based democracy, our language and history. He can't have read the "Britishness test" for new citizens on which I commented recently.

He also acknowledged that critics mustn't "play into the hands of the BNP" and talked about fairness and the alienation that exists among some members of the white working class. For a man who has delivered a lot of sermons in his time it wasn't clear to me quite what he was trying to say except to express unease.

He certainly conveyed that. Must try harder.