NGK do 3 types of plug for the zetetc:

PTR5A-13 - Platinum tip now replaced by PTR5A-10
PTR5A-10 - Same plug but with 1.0mm gap cos the 1.3mm gap ones weren't lasting.
TR5A-10 - Standard copper core plug. Cheaper but wont last as long.

Platinums about £5 each Copper about £2.50 each.
Bosch leads probably as good as it gets!