Can you give ne a rough idea of what you spent £20,000 on please. How long did you extend the lease by?
Take a look at HS283 for information on CGT and the main residence and here for information on working out the gain or loss.
I would claim all the refurbishment costs so long as none of the expense was claimed against rental income. The lease extention costs can also be claimed. You can also claim the legal fees incurred on purchase and sale, stamp duty, survey fees, selling agent fee, improvments costs (an extension for example).
I hope this helps but let me know if you have any further questions.
Would you mind rating my answer before you leave the site please.