Keith,
Thank you I will explore the NGK C7HSA and C8HSA and their iridium equivalents CR7HIX and CR8HIX. Thanks for the offer of the used PTENL80, however, even when pressure tested satisfactorily, used plugs often do not perform well.
Jim,
Not sure if your comment is tongue in cheek - this is the standard Bristol plug spanner, supplied by A.C. with every Bristol engined car and, yes, the UJ is taped up which gives it a little stiffness and makes finding the plug 'ole easier (the more raffish amongst the audience will realise that this sentence reads better if the word 'plug' is omitted......)
Barrie