Apparently there is an automatic block on all AIM shares when attempting to trade through an ISA. This is probably the stock response from brokers. Fortunately at Waterhouse they seem to be a little more clued up about the rules for trading within an ISA and have the ability to un-block the trade, where appropriate.
DES is tradable in an ISA because it is listed on foreign exchanges. I believe it is listed in the US as well as Frankfurt. This appears to be some loop-hole, but I'm not complaining !
I'm funding my wife's ISA account tomorrow and we're making a further purchase. Let's hope the price doesn't run away too much in the meantime.