In reply to FesteringSore:
If your valuation is approaching accurate, they're taking the piss. I would flat out reject it. One possibility is that they're actually angling at about £410,000, but have taken the attitude that if they put in a ridiculous offer to be rejected, if they come back at £400,000 they're most of the way to "meeting in the middle", but if they came in at £400,000 they could expect to be haggled up to £430,000. So make sure you've got a firm lower limit.
Also - £100,000 of alterations? Are they gutting it down to a shell and rebuilding it? The deal with a house is you can take off the amount you'd spend on essential modifications (the kitchen is from the 1970s / the roof needs maintenance) not "would be nice" alterations.