So I'm back in the game. My problem had a combination of causes but the ultimate symptom was poor wrist position. Essentially - too often my wrist was cocked downwards (towards my forearm) so that when delivered the seam was straight on to my imaginary stumps position.
So what were my causes?
The easiest one to identify was not standing straight during the delivery.
I also initially worked at making my delivery stride more side on so that my arm came around more. This definitely helps but its not something I feel comfortable with at present as the twisting forces through my torso and shoulder feel unbalanced.
Finally - and counter-intuitively - I stopped attempting to 'try and spin the ball'. This requires some explanation and its worth pointing out I'm fully expecting to revisit this as I get the rest of my kinetic chain in alignment. When I bowl off-spin, I get decent turn without actively trying to spin things. Simply by getting the grip correct and the right follow through, the ball seems to just 'flick' off my index finger. If I try harder, I get the sensation of giving it a good rip and it turns more. In other words, I just did what 'felt' like it worked (yes I'm a kinesthetic learner). But when attempting to leg spin, I actively try and perform the wrist action to make the ball spin. My change here was to allow the ball to spin naturally out of the hand. With the right motion, the ball just seems to grip and flick off the ring finger and spin anyway. By stopping trying to spin the ball, I could isolate the wrist into a better position for when the ball was released so the seam was at a better angle.
Now this isn't the end but its a good start as I can see some success here. It is apparent that I'm not getting my maximum spin and I know I'll need this extra spin to get a bit of drift or I'll get smashed around by good batters charging down but thats something I'm going to work up to. For now working on consistent spin with reasonable accuracy (avoiding wides or long hops) is my aim - and it finally feels achievable once again.