same thing happened to me with adrian flux a few years back.

I got a quote over the phone and listed all the mods and took up the policy, then when i recieved the paperwork half the mods werent on there. So phoned them back and told them, they said "just edit it with correct mods on the paperwork". Then a few weeks later i got a letter saying that i owed them a further £120 for undisclosed mods at the time of quote

I fought it out with them and the only thing that they siad was making a difference in price was the fact the car had non standard springs (lowered 25mm i think) therefore it was £120 more.

After lot of phone calls and fighting with the supervisor/manager i got it fully covered all mods for NO extra

But I WONT EVER use Adrian Flux again no matter on what price they quote for the RST (or any car i own )