Chris - I have a 1.8K SV 03 plate
I changed the original Avons to Toyo R888 205 45 ZR 16 in 3G soft/medium compound which improved the roadholding in the dry and made a huge difference in the wet.
I tried 13 inch for a short while, but whilst there was an improvement, I found I hit to many things with the sump or gearbox, so went back to the 16s and fitted the Toyos. I don't track it, but drive fairly briskly and would recommend them for fast road use. They don't wear out that fast, either.
Try putting it into the search function - you'll find lots of advice!