French state-owned power company EDF has been given permission to start the pre-construction of "Hinkley C", the third nuclear power station on the Somerset coast of the Bristol channel.

From the announcement, it might appear that the firm will just send in a few bulldozers and put a fence around the site, much like any other construction project.

Wrong. EDF must completely clear 400 acres of land and will then move more soil and rock from the site than has been moved to create the Olympic games site in London.

There's also the small matter of planning permission. EDF still has no formal permission to construct the reactor, so this would be a bit awkward if Centrica, its partner in the project, decides to pull out at any stage (as the City is advising it to).

They would leave one of the biggest holes in Britain.

But the most extraordinary thing in this whole saga is that the companies are going ahead without knowing how much this 1,600MW power station will cost.

Its prototype are the two being built at Flamanville in Normandy, France, and in Finland. Both of these have taken nearly twice as long to build as expected and, at £5.2bn for Flamanville, are turning out to be nearly twice as expensive as expected.

Since Fukushima in March, the costs have risen further. Stress tests to determine whether these reactors can withstand natural disasters are expected to put the cost up hundreds of millions more.

But anyone who thinks the Hinkley C or any of the other new generation of plants are a done deal despite what government, the industry and and a few climate change greens want, should think again.

Here's why:

1. There are real fears that the design is compromised and may not be used in Britain, or in India, which has plans for six reactors of the same design. The head of ASN, the French nuclear safety authority, went on the record only a few months ago saying that he "could not rule out" a moratorium on the third generation European Pressurised Reactor (EPR). The shares had to be temporarily suspended.

2. Centrica, the energy company which operates most of the UK's nuclear plants in partnerships, is far from certain to stay with the project. Sam Laidlaw, its chief executive, last week said: "The reality is that [after] the news about Flamanville and Fukushima we are clearly going to have some changes.

"People have been focusing a little more on the optionality since then. It's by no means a done deal. We'll give it our best shot, but if at the end of the day, we add all that up and we just don't think the returns are there for the perceived risk, then we've always been very clear that this is an option, not an obligation for us," he says. "We will only do it if it makes good sense and good returns," he told the FT.

The awful reality for both politicians in love with techno-fixes, and advocates of nuclear energy for climate change reasons, is that it really does not matter how good their argumentsin favour of nuclear are. It is the City that decides if the reactors are built. It was the City that ruled out new nuclear plants throughout the last 20 years when there was no law preventing their construction, and with the price rising fast, and the age of austerity with us for years, it will be the City that decides this time, too.

I would say it's still only 50:50 that west Somerset will get Hinkley C. But if it doesn't, think of all the things the locals will be able to do with a vast, 400-acre hole. No prizes for suggesting that they fill it with rubbish to generate energy from waste.