Little bit of an update and I have to say I'm proper baffled by it now

Have redone the breather system as per the Mk3 setup (cheers RJ) just to rule that out as the current setup was a bit bodged. Looks much more tidy I have to say.
Went for a good blast around the countryside with a mate to check the smoking. I can confirm it is literally on gear changes at high revs, not all of the time having seen it from following behind. It doesn't smoke constantly on power, just a fairly large plume at higher rev gear changes when coming back onto the power which lingers for a few seconds and clears. Idle seems clean. It's definitely not down on power.
To be honest I'm leaning more towards it being a piston ring issue however I'm still hung up on this good compression result I haven't been able to see the head builder however after a discussion with him, he recalled everything going back together tightly during the rebuild and as it should.
I'm pretty stumped to be honest and I appreciate the help I've had. Not really sure on next steps. What do people think - is it worth stripping the whole thing down again and sourcing another block?