So, the Boogie is back in my studio and as Russ indicated, it certainly wasn't a well amplifier, it sounded really quite unwell and gut feel was that the problem was a lot more substantial than a pre-amp tube.
But, start simple and small and move towards large and more complex is the rule, so that's what I did. Luckily there's a pretty substantial collection of tubes here and as a few people suggested above, the pre-amp tube was the obvious starting point and bingo, it was also the culprit. So it's all fixed now.
I have also recognised that fate has decreed that I shall always own at least two pieces of Mesa Boogie amplification
Russ/@binky_bass, many thanks for your patience throughout mate, as it turns out it was after all quite a simple fix although we weren't to know that from the symptoms, and also as I said above, it really did feel like something so much more problematic than the pre-amp tube.
Gonna need to buy some more electric basses again now