Thanks for your very helpful suggestions. I will need to get my mechanic to check for your suggested solutions before I can confirm that the problem is solved. I will get back to you after that and advise the outcome.
My local mechanic has checked both of your suggestions but can nothing that rattles. The heat shield is solid and tapping the exhaust canister has shown no noise. He is quite puzzled.
The only other thing I can add and its not easy to be exact when driving [ the only time the noise occurs ] is that the noise appears now to emanate from from behind the drivers side in the vicinity of the rear drivers side [LHD ] wheel. Its still intermittent and on the whole doesnt happen when driving on a smooth road. Any further ideas ?
As a non expert, the noise which is quite loud at times feels like a stone rattling in a metallic jar or 2 metal plates banging together.
by jacking the car up so the wheel is off the ground and shake the tyre from top to bottom and from side to side (with someone holding the steering wheel). There should be no play in either direction.
Any play in 1 direction will usually require a suspension joint to be replaced, if there’s play in both directions then the wheel bearing may be at fault.
Worn drive shaft joint - typically these won't produce any vibration unless badly damaged but they can be noisy or 'clonk' for a long time beforehand, turn the steering to full lock and look at the drive-shaft rubber boot there should be no rips or tears in it and the suspension should be clean and dry of any grease. If you reverse at a reasonable speed with the steering on full lock a worn joint will 'click'.