Basically, don't use too large an objective lens or magnification. 10x magnification and 44mm objective should be your maxima. Scopes like the MTC Viper 3-12x44, Bushnell Legend 5-15x40 or Bushnell 3200 4-12x40 are all popular. I suggest searching for Sparky's HFT kit thread in the main airgun section to see what people are using. Also get yourself up to Lea Valley on Saturdays, especially the third each month to have a go plus don't forget West London Rangers at Harefield every whenever it is they meet.
Jonathan