When you're ragging the car on the track, the heat build up will cause the oil to vaporise and get into the cylinders thus reducing the octane level of the fuel causing detonation. As mentioned above, use Maxol E5 and some booster, Lucas or NF. A knocklink would be a good idea for the track anyway if you can afford it, that way at least you know if the engine does start to det.
No need to run E5 all the time on the road if the car is standard Irish turbo mapped for 95. Just overkill IMO...