I stripped it down last night and before i remove the barrel (this is not standard No1 Mk111 ?)
I'd been wondering about the barrel profiles of various Mks of these transitional rifles and trials rifles.

I'd been told that there was no difference in the MkIII barrels and those for rifles like yours and no difference between the No.4 barrels and the No.1 Mk VI barrels.

A friend has a Britishicon No.4 that has a barrel that is far slimmer than any other No.4 barrel I've seen.
It doesn't appear to be a Mk VI action but I'd wondered if the barrel might have come from a trials rifle.