I've had ins with them for a couple of years, although they have repeatedly been slack with the paperwork, they do know what they're on about, and give very good prices.
They do allow monthly payment, with the first payment being 2 months? If I recall (always paid yearly) with 10% Extra for Monthly payments (They may have changed that by now though)
I was able to list all my mods, and specifically list my alloys etc.
They also agreed to a fixed value, rather than book value, although I did have to point out the cost for my extras.
I went with a larger excess to drop the yearly price, which has worked well for me....
Not sure about your age tho.. they didn't used to touch < 20 Y.O. - But times change......
Any insurer will gouge you for your age, it's pretty much unavoidable, but if you pay through the nose for a year, it will drop pretty quickly... and also play the insurers off each other when it comes to renew after a year, and you'll see a good saving...
You just have to take the first big hit, if you want your toy insured!
p.s. AAMI are by a large ammount the cheapest for GreenSlips, when I re-upped this year.... Quite surprising!