In reply to Rigid Raider:
It's lower back and not chest or upper back so not ribcage. From what I've heard ribcage is unmistakable agony when you cough etc so thankfully not that.
I really liked Rog Wilko's suggestion as it sounded so pleasant but it was actually really unpleasant - my lower back really did not want to relax onto the floor and it stiffened up.
Dax H suggested touching toes & stretches, done that very gently and feels good.
Hopefully back on the bike next week, although my wife is worrying about traffic dangers
Andy - thanks, looks good.
Thanks all.