Just heard back from my specialist insurance company (Airsports International - AIB Insurance), they currently insure me for all of the cars.
1984 Scirocco Storm - heavily modified, agreed value etc. ~ £129
2003 Boxster - Won't do on classic car policy yet, as not quite old enough and I haven't owned it long enough - however, not bad at ~ £299, limited mileage
Pretty much any A2 (ie both my current runners) - not old enough for classic policies yet, they're saying about 20 years as most are, but ~ £269 each for the year.
Very good to deal with, and matched my single NCB across all the cars. I've been with them for about 5 years now, it's got to the stage where I know most of the front desk staff and when I call they ask "which A2 are we changing this time...?"