The reason they gave me for the 18% deduction was that their obligation is to put me in the same position I was in before, and if they were going to settle by cash rather than through their preferred supplier it would cost them more. Make a big enough nuisance of yourself and they’ll cave in though ;o)
I also spoke to them about proof of ownership in the event of theft (given that I don’t have receipts for all of parts/some things are second hand) – they just said to take photos of all bikes, keep receipts if you have them (or other proof of purchase, e.g. credit card statements) and ideally have a list of all the components as well.
They’re still good – just a bit harder work now!