“What you don’t understand, Chris, is that no Tory MP can survive on £33,000 a year,” one of the grander Conservative MPs (and a multimillionaire to boot) remarked to me as we were queuing in the members’ tea room. Much to my embarrassment, he said this within the hearing of the serving women, whose wages were about a third of ours.

The year was 1995. Since then MPs’ salaries have more or less doubled and the allowances are a good deal more generous. Even so, it is apparent that many Tories struggle to survive on what most of their constituents would regard as an income beyond their wildest dreams. The departure of Mark Simmonds is only the latest evidence.

It is not as though Simmonds was festering on the backbenches. On the contrary. He held one of the most fulfilling jobs in government – Africa minister at the Foreign Office. I know because I once held that post. Not many of us have the privilege of doing work that can make a significant difference to the lives of the poorest people on the planet, but this was one of them. Only the other day, he was chairing a meeting of the UN security council on the Congo.

His ostensible reason for departing – that the expenses regime is insufficient to allow his young family to live or visit him in London – is puzzling. He didn’t have to live in hotel rooms three or four nights a week. The parliamentary cost of living allowance entitled him to claim up to £27,875 on the rental of a second home, either in London or in his constituency.

Although he attributes his departure to the alleged inadequacy of the expenses regime, my guess is that he was struggling to get by on a family income in excess of £110,000 a year (a £90,000 ministerial salary and the £20,000-plus earned by his wife for managing his constituency office). Which brings us back to my original point: many Tories find it difficult to survive on the salary of a backbench MP or even a ministerial income.

As most of us realised long ago, contrary to what is sometimes asserted, we are not all in it together and never have been. If you live in a world where your outgoings include a home in one of the leafier parts of the country, private school fees and perhaps private medical insurance, then an income of £90,000 a year, let alone the £67,000 salary of a backbench MP, does not go far.

In fairness (not that I wish to be fair) there is an issue here, given the vast and growing disparity in house prices between the home counties and the rest of the country. One only has to glance at the electoral map of the country to see that Tories by definition represent the relatively prosperous while Labour MPs by definition tend to represent the less prosperous.

As a Labour MP who for 23 years represented one of the poorer areas of the country, my family and I could live well on an MP’s salary. With the exception of perhaps one or two doctors or a headteacher, I earned more than any of the 500 or so members of my local party. My Tory equivalent, representing a seat in the home counties and without a private income, is likely to be one of the poorer members of their constituency party. In a party where status counts, that must be painful. Which is why so many Tories feel obliged to top up their public service salaries with outside earnings.

Until he became a minister, Simmonds, for example, earned a useful £50,000 a year as a strategic adviser to a private health company. He also had other outside earnings. For him, going into government no doubt involved a substantial cut in income. For most of us the privilege of serving would more than make up for any temporary loss of income. Not so in his case apparently. He has not done himself, his party or the political class in general any favours by the manner of his going.