Adam, if you look at the G-INFO UK register database on the CAA website, and search for a Lake as a sample eg G-BASO you will find the minimum liability insurance required by law for this aircraft for non-commercial operation is shown as an estimated £3,228,720 (specifically 3.3M SDRs which is a contrived 'basket' of currencies). I suspect this may be where your form fell short - by nearly £3Million!! Hull insurance is not mandatory, so £0 there should not have been a problem.
Personally I have insured my machine with Traffords for some years; I am pleased with their price and admin service, but I've not had to claim and I hope I never will.
Slip