Dave has it more or less. You are rightly a legend on here and we can"t do without you! Thank you for your time and effort again.
Yes, the hose between the intake manifold and the oil bath air cleaner was collapsing in on itself. But when the engine was not running it regained it"s normal shape.So you had to be off the tractor with the engine running to see it. And to look in the right place! Air filter always kept clean so not excessive suction due to blockage there.
It is a newish hose (less than 2 years old) but sourced from a well known discount spares supplier. It was a poor fit even initially and I had to slightly modify it! Obviously unsuited to the negative presure; designed for coolant use I would say.
Being a poor small holder I have left it on but reinforced it with a solid tube (in the collapsing section)on the inside for the moment.
Posted it on in case someone has similar problem.