Gutted, and Will keep my yes open also.
As for assessing value, you probably best get a licensed "loss adjuster" or "beëdigd taxateur" in Dutch. I have some experience with a classic car valuation, and notably the assessment involved a list of ebay, auotrader and marktplaats searches. Quite specific searches too. You could start with the build list and work the parts prices.
Notable difference is that I got the car valued for insurance purposes prior to any event. The independent confirmation that the bikes were actually in your shed and the verification of their condition is missing. Do you have purchase records in eBay or paypal or even receipts?
And please beware that knowing your loss though helpful, does not give you any rights with the insurance. The licensed taxateur or your insurance agent should advise you on your position.
Good luck