From my research on the subject (not stating as 100% fact, just what I've read) the D2 at launch didn't have type approval for seven seaters on coil springs. Later on however they did gain type approval, so some rolled out the factory with seven seats and coil springs.
Land Rover part numbers RKB000340 & RKB000350 are stated as "SPRING-COIL, GREEN / GREY, REAR, RH/LH, HEAVY DUTY, 7 SEATS, RHD"
You've told the insurance company so done the right thing. Most don't give a **** anyway. We've got two D2s in the family, me and my Dad are both insurance brokers, both D2s are seven seaters converted to coils and it doesn't bother me one bit.