Thanks for your replies.

the legal charge says that the value of the unextended house should be determined by a qualified valuer. My ex does now want to move down that route (whereas before she wanted to take the alleged high building costs off the market value of the extended house as being the value - there's a surprise !)

As we have to instruct the valuer jointly, I've asked her solicitor to go ahead and draw up the instructions and I will review and add my own instructions - this sould be interesting. I can see the agreement of the instructions being a lengthy process !