From memory, the Douze was 4,500€ excl. VAT back in 2015 or 2016.
Prices have now doubled and the exchange rate is appalling however knowing how insurance companies work, they'd likely ask for the bill of sale then apply some depreciation so if anything, £3,500 is over valued
You might want to check to see if they apply average/underinsurance.
They may reduce the settlement in the event of a partial loss claim.
( Assuming the Douze is worth more than £3500)