P.S. I mean the various UTube videos - not the basic ones on the download page

Sorry!
Of course you are quite correct, and I can only apologise for what was indeed a stupid error.
In my defence, I was having terrible trouble yesterday getting the page to load properly - and to find the downlad file at all - so I missed the obvious!
I still haven't got any of the 'help' videos on OSC to play at a resolution that enables me to actually read anything on the screen - so I was battling with a whole host of 'unknowns' (to me!) at once.
Now I'm trying to get my head round 'intersect' - as that may actually do what I was trying to achive. We'll see.
As I only started on SU 2 days ago, perhaps you can forgive some initial confusion.

Thanks very much for 'being there' and willing to help - even if.....

I've made some progress!
Just re-installed SU8 only adding the OSC plugin (left off the Fredo and Eng toolbox ones I had used). That made no difference!
I have a simple test file with two objects - the standard box and cylinder.
I had turned each into a 'component' - perhaps that was my mistake?

I have just made each object a 'group' and 'Eureka' - it now seems to work.

Perhaps as a new user I have just made a stupid error, should it work with 'components'?

I'll have to try some more - but thought it worth noting my modest little 'success'!

I actually omitted to say that I have opened the Roiby Console - although I admit I don't understand how to use it.
It just shows two empty boxes whatever I do - no error messages, no nothing!

Can anyone help?
I am new to SU8, have loaded OSCoolean - and have the toolbar active...
BUT when I select one of the OSC icons and then TRY to select my objects, I can't - it just will not let me select anything!
I have 2 simple test objects - much as shown in the videos.
At present I'm completely stumped - just can not work out how to move on from here.