Okay this is what its not. Its not the springs, its not the engine mounts and its not the lower suspension linkages.
The sound only appears during a long journey, and is most noticeable during motorway stop/start traffic when taking up and letting off the gas repeatedly. There is no noise when maintaining a constant speed or when cold on local runs.
The dirty bits have been checked and nothing seems to be loose, falling off or doing anything it should'nt, but still there is this annoying noise
Any ideas cos I've run out.

l realise you said it is not the engine mount, there is a mount/stabilizer underneath the gearbox which attaches to the subframe. could be that!

Anyone lost a cat ?

Could be driveline shunt.

Driveline shunt is due to the small amounts of play through the transmission system - each union joint has a tiny amount of play, each cv joint, each gear mesh, including the differential, & finally the wheel hubs. if they were all tight, there would be no drive line shunt at all, but in practice every one has a tolerance.
Even if none of them r badly worn, if there r a lot of gears/joints etc in the system, then these can tot-up to give a noticeable ''clonk'' and/or lurch when powering on / lifting off.

Of course, if one or more is worn, then it is even more noticeable.

l had this problem on a y reg
scenic unfortunately l did not get to the bottom of it, but it sounds like that you have no shock
absorbers on the front, sorry l can not be of
more help.

l was going to say the dead cat one, but otherwise its the ghost inside the machine!