I don't mind admitting that I can be a bit slow on the uptake but eventually I get there.
No matter how many times I read manuals and looked at diagrams I just couldn't "get" this shimming thing so I decided to experiment with one of the spare cylinder heads I have. Terribly boring for those who know what's what but I followed the workshop manual and removed the rocker shaft assembly and the camshaft. Then it was blindingly obvious what these "shims" are - little discs of steel that sit on top of the valve stems so depending on how thick they are (could be very thick like me!) they affect the valve clearances.
Also the famous "buckets" now mean something to me - one is visible having been removed to the right of the head, next to the ratchet. This sits on top of the inlet valve stem with a "shim" in it.
Apologies to all those out there who know what they are doing but you must have come to know how mechanically challenged I am by now! Well, I know a bit more now so it's on to proper shimming shortly (but this will be delayed by a trip out tomorrow which is also Triumph related).