How much capital gains tax will I have to pay if I sell my flat?

Hi. My wife and I bought a flat 8 years ago for £225k. We've lived in it until now.

We've just remortgaged it as a buy-to-let, on the basis of a £400k valuation (crazy London market). We've used some of the extra value to buy a house (with a big mortgage) that we've now moved into.

My question is: if we decide to sell the flat in a few years, how much CGT would we have to pay? For the sake of argument, what if we sold in 2 years time for £450k?

I had thought that if we sold within a certain period we wouldn't have to pay anything, as it's been our main property till now. But now I'm not sure!

Advice would be welcome.

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