Thanks, I messed up because my 'hole cylinder' wasn't quite cutting right through the main surface and therefore messed up when I tried 'Boolean>Difference'
This is the first boolean that actually Did work!
Sorry for that. will check properly before posting in future.

[Plus I was sort-of expecing it not to work because of the other failures]
Have just read your reply to my Oscilloscope problem, hopefully I will be OK now... Thanks
Scraggy