Thanks for our your response Bernard, it has helped me enormously.
The stressed component suggestion is possible but I have two Transformers exhibiting the same characteristic, maybe the same component is being stressed, but probably unlikely.
The price of SMPS is surprising that they will probably be cheaper than the old wound transformer thing but on reflection very probably as a wound transformer set up would also require some form of protection increasing the unit cost, so your suggestion is almost certainly correct.
Thanks again for your very helpful feedback, it is more appreciated than you probably realise, Thanks again.