Just popped down to Circuit Supplies for some rear DS2500s and found them most pleasant to deal with. They're the main distributor in the UK for Ferodo racing pads and they keep both pads in stock (I had been expecting to have to order them).
They quote £53-odd + VAT for the rears, although I got a discount for collecting them rather than having them delivered. They opened up an AP brake kit to show me what it consists of and gave me a run-down on the various pad materials, including showing me a pad where the heat-insulating layer between the friction material and the backing plate was visible. The DS3000 are not recomended for the road, although apparently they will withstand a high enough temperature without fade to melt the surface of a brake disc on a Renault Clio.
The drawbacks of DS3000 are that braking performance is less progressive than the DS2500, they need a bit more heat to start to bite and they eat discs (braking with DS3000 "looks like a catherine wheel", apparently). For smaller cars (like the aforementioned Clio) they find that lap times are actually
faster with DS2500 than DS3000. I asked whether the 1.5 tonnes would be heavy enough to warrant the use of DS3000, but they didn't think so if it were to be used on the road as well.