I've just taken out a multi car policy with Aviva for my Z4MC and the wifes fiat 500. Quotes were around 550ish with protected NCD and legal cover for the Z and 300 for the fiat. Ended up getting both for £711, which I was really happy with. Admiral were around 80 quid cheaper, but when I went to take out the insurance they wouldn't cover me for the £4000 of extras on the car, (wheels, exhaust, carbon intake). This was really strange as when you are on the website doing quotes, there is a drop down menu with maybe 40 different options for modifications to the car. I ticked the 3 which applied to mine and added the value. When I rang up to go through it with a fine tooth comb, they said they would not cover me for the mods...but I was right to declare them. They said as I had told them, I would be covered for the replacement standard car, but not the cost of the mods if it was written off. If I hadn't told them about the mods, wrote it off and they found out, the policy would be void. So, if you have any mods at all, don't go with admiral.
Did exactly the same with Aviva, and have everything declared and covered....great!