Cracked frame & tubing cracks - any suggestions?

So, the noise stops if you "hold the top tube and push downwards", presumably this takes your weight off the saddle? If it's also silent when standing... isn't it more likely to be the saddle rails/post than a frame crack?

I find the rear bolt on my Thomson post needs tightening every few months to stop the creaking...

Your diagnosis would indeed be correct Dr Munch. The seat post in the frame was twisting and creaking. Removed post, greased it and re-installed, did up bolt nice and tight. Problem licked. Suppose I should not always assume the worst then, eh.

prblem is on steel frames the sound bounces around all over the place so it's very hard to tell where it's coming from...

For sure.

I think I may have actually found a 5mm long crack at the back of the head tube where it meets the top tube. Could be a crack in the powderocat though. Time will tell.