Well the new cable came today, plugged in & ...same problem.

I did discover that , with the cable plugged into the amp , not the guitar, touching the open cable end to the central heating pipe did this:

* touching with the part of the cable end furthest away from the tip (ie about an inch from the end of the tip) the buzzing dissapeared (ie the result I had earlier).

but..

* touching with the tip of the cable end made things even worse. much more buzzing & cracking sound.

Tried it in various rooms in the house, in the garden on an extension mains cable, same problem.

Next : used batteries instead of the mains power supply, no guitar plugged in. This time the buzz is still there, but touching the tip of the spare end of the cable to the cental heating pipe

results in the buzz getting worse.

Dont have access to another amp to test.

I read these clues to mean the amp itself is faulty, would welcome any other ideas before I contact the supplier.