Captain Cotswold.... Do you have a Halfords trade card or know someone with one? If so go and get NGK BCR8ES plugs and gap them to 0.6mm just to try them. The plugs are less than a fiver on trade

Had bother with 071's in the past and I always use the cheapy NGK's gapped at 0.58 now and get no bother. I run greys and 34psi on a T34.63.