Thanks Chris - I guess I have a worry that this could be the issue. If that is the case then there will be no question about what I will need to do. At the minimum it would be take everything off the front and at least renew the tensioner idler and piston...without disturbing the timing!
But, I am back to head scratching today...cold start and the belt seemed to have gone back to flapping 
. Stopped the engine without it warming up and felt the belt...a bit slack again.
So I will get the mirror and torch out and have another look at the tensioner piston. If it has retreated once again then I assume that the spring is weak and unable to overcome the force required to tension the belt.
Will keep everyone posted.
Jonty