I ve got a mk1 Gti 1.9 too
Im insured with Adrian Flux with ALL mods declared:
Repray in yellow (used to be red)
Bored out to 1.9
Performance Cam
Performance air filter
Four branch manifold
Full supersprint stainless exhaust
Lowered
Uprated brakes
Non standard alloys
delocked boot
All plastic trim removed
Single headlight debadged grill with grill spoiler
Cat 2 immobiliser
21 years old four years no claims and clean license for ?545 which I thought was pretty good considering mods and age. no else has come close. None of the internet ones will touch me and I ve tried other places who quoted in excess of ?2000!!!!!
Closest other was HIC at ?700
Had to send photos and fill out a modified car report but they seem pretty good at adrian flux. Try them. If you can try and get an agreed value rather than market value because thats so vague.