Just done a quick warm up today (pre-gig) - and did my stretches before hand (Cheers DrSmaggs)
They helped a lot, but I also made another breakthrough (which is bl**dy obvious now I've realised it)
While concentrating hard on relaxing my left hand, I suddenly found I couldn't hear what I was playing - I turned the bass up and suddenly it was easier to relax the left hand and play more 'gently' - which ends up being much more relaxed.
I try and keep my bass amp fairly loud and play softer with my right hand (after I did a gig with someone else's rig which was too quiet, and ended up with blisters on the tips of my right fingers

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Now I need to make even more effort to turn the bass up and play softer, and relax more, and maybe this will help my left hand problems as well.
(The stupid thing is I had almost exactly the same problem about 3 years ago playing electric drums with hard rubber pads - if your amp / monitor isn't loud enough, you spend the entire evening smacking the hard rubber pads at full power, and end up with very sore wrists the next day - it just never occured to me that this same issue could be causing my bass problems).
Anyway, I'll let you all know how I get on after tonight's gig (unfortunately it's a garden party - and the weather's not looking so good

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Cheers again for all your help and ideas
PaulF