Soldato
Is he RICS chartered ?
I won't lie, I read the title as Payment dispute, onions needed.
Ask him for a breakdown of how exactly he's come to that amount?
As you say, he should be compensated for the missed appointment, but I can't see that coming to half the total quote when he hasn't actually done any work?!
Edit: how did he "tell" you to "go ahead with it anyway"? If it's in a provable format, e.g. a letter or email etc., then I wonder if that would contravene the code of conduct of any professional body the surveyor is a member of. Would be shame for him to lose his professional status...
To be honest I'd generally take a 50% deposit on any contract before work begins, which means I still recuperate funds for wasted time if something outside of my control forces the work to not go ahead. I'm not a surveyor, though, so I don't know how he has come to that figure.
Did you sign a contract with him when taking him on board for the work? He should really be providing that, with the stipulation of a percentage or minimum fee regardless of whether the work goes ahead. Without that, he can huff and puff all he likes but he can't enforce anything without providing you with a breakdown of his incurred costs so far.