Success. I first tried the "Fazerider" method of checking the clearance through the gap with the cover lifted as far as possible. (It did remind me of the gynaecologist who decorated his hall, stairs and landing through the letterbox). Obviously it must be possible, but after checking number one cylinder I decided to have another go at getting the cover out of the way completely. The breakthrough came by doing as "Unfazed" suggested and loosening the throttle return cable at the carb end and removing the thermostat housing bolt to give just a bit of movement. After some more juggling and pushing out came the camshaft cover.
The final result was that all the clearances were within spec, with just a few of the inlet clearances apparently having opened up by something approaching 0.001" (0.025mm) since when I last checked. In the end all I have achieved is piece of mind and the satisfaction of actually getting the cam cover out of the way.