Sorry to be so long posting an update but life has got in the way and I have only just completed the repair. Fitting a new head gasket seems to have fixed the problem, so many thanks to those who suggested this was the fault. The head has been overhauled and hardened exhaust seats fitted - maybe unnecessary but cheap enough to do while the head was off.
My concern over bottoming of the head bolts was not misplaced, the head had been extensively skimmed in the past and the centre bolt on the nearside was too close to bottoming for comfort, so I refitted this with a thicker washer.
Many thanks for the help and advice, on now to the gearbox oil leak and beyond!
It’s good to know that the forum is there to help, I’m sure I will be back.