GCT - PPR conversion - valuation evidence

[njt]

We bought a house in January 2008 and have lived in it as our principal residence until May this year. It is a jointly owned freehold with a mortgage. We have been trying to sell the whole house without success and so now plan to convert to two separate freehold residences in the hope that it will be easier to sell two smaller properties than one very large one. We have been told by at least 3 estate agents not to expect to make any money on the conversion (on the basis of that the costs of conversion will equal or exceed any added value). However, they may be more saleable.

I read some other posts in relation to capital gains. Could anyone advise what form(s)s of evidence I should seek to establish values before and after conversion, if necessary? Is it a case of getting estate agents round to give before and after market appraisals?

Thanks

[section 44]

Obtain evidence of sale (as opposed to advertised) prices of any comparable properties.