I think I just might have embarrassed myself big style. I spent quite a while under it yesterday trying to find anything that could be the cause and found nothing at all. So decided to rely upon the upcoming MOT trial run to find it for me.
However, this morning when I finished work and went out to it in the car park, I took off my clutch claw as usual and put it in the space between the seats where I keep it. It slipped out of my hand and struck the bulkhead behind the seats and made the EXACT noise I've been hearing. This is when I realised that when I was tidying up the inside a week or so ago I had hung the cloth thing that is dangling from the bulkhead behind the seats over the top behind the bulkhead. So I tested it out and I actually watched the clutch claw tilt forward on heavy ish braking and come to rest against my Eberspacher, then as I accelerated away it fell back and thumped off the bare bulkhead, the noise, I am positive, has now been identified. I feel a proper ****T, but am promising myself I'll rule out loose items the next time I hear anything like it again.
Assuming that this was the cause, I wish more "repairs" were as easy as this one!