Cheers Broady! I sorted the wheel knocking issue and fortunately it was more obvious than a bad bearing. Turns out that the o/s rear wheel nuts were not fully torqued down which was allowing the disc brake adapter (for lack of a better term) to slightly shift with each rotation and hence the clunk. Sometimes it is just obvious stuff!
I took the fasty out over the weekend for a longer ride having also readjusted the valves, which were a smidge tight across the board. Travelled out 30 miles from Bristol to the Cotswold Airport and had the unique privilege of being able to park up under and next to this colossal 747:

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It was a great run out, car pulling well and running smoothly. On the journey back, however, I started to experience a bit of hesitation on acceleration, causing a slight kangarooing motion. I drove on hoping it was an anomaly, but soon I started to notice decreased power when going uphill or accelerating from a stop. It also began backfiring (misfiring?) when not under load heading downhill. Not wanting to push the car, I pulled off into a nice country pub and had a cold drink whilst the car rested. When it came time to restart the engine it just wouldn’t take. The starter was turning over freely, but the engine just wouldn’t catch. I ran through my loose diagnostic procedure: checked fuel, points gap, electrical connections and plugs. Still nothing. However, I did notice that the engine was hotter than usual, so I left the engine hatch off and the boot lid open and gave it chance to properly cool down. After another 15mins I tried the key and it sprang to life, phew! I seemed to have full power back and it got me home the last few miles.
I took it out for a spin yesterday and same thing occurred, only this time I probably only covered 5 miles. As soon as the engine is switched off it does not want to restart until fully cooled off. So clearly there is something amiss or an engine component is not coping with the heat. The car is getting fuel though as I whipped off the tops of the carbs to verify that the float chambers were not empty, so I do not believe this to be a vapour lock issue.
I need to work out what is going on, so will check the timing in the first instance. Maybe the coil is on its way out, I know that they can be affected by heat. Maybe my valve adjustment has thrown things off, although I was careful to ensure everything was accurately set to 0.15mm. It will be a bit of process of elimination. Can you all think of anything else I should look into? Anyone encountered this issue before?