and it's not true that worn road tyres are good on track, they don't behave like slicks because their compound is much harder, they behave like tyres that rely on tread block movement for grip without tread blocks! And if it's damp, forget aboud it.
Give Yozza a shout, I am sure I have read on other posts that they have trckday tyres in stock.
Also have a look at the RS2 from Hankook, come recommended by one of George's hill climbing buddies and only £60 a corner from mytyres, and they were on £56 a corner recently...
George is running the Kumho V70's, which are pretty pricey at around £120 a corner.