Two guesses what I've been doing today...
... fitting, refitting, adjusting and readjusting my harness then putting it all back on the twinset.
It worked a treat, I can now reach the valves and manifold with ease. I just need to do the stretching exercises to improve things and then get back in the water to finish off Tec Deep and Tec Trimix
Thanks Garf for an informative thread