Not going to say who I'm with as I wouldn't recommend them (currently having a, ahem, 'discussion' over their response to a claim on the house for a roof leak - long tale about 24 hr helpline closing at 9pm, and then subcontractors saying first 48 hours and then that becoming 48
working hours and then, and then...aaaaarrrgh). They did turn a minor annoyance into a crisis.
For info just did an alternative quote on a few well known places and they all seem to have it. Try Aviva (comes up first in my google) - click modified and there is a list of stuff under modifications.
EDIT : Just checked with a workmate who has loads more mods and he uses Bell Insurance. If you do it online there is a big list of mods including a non-standard air filter.