Well I went and had another look at it, ruler in hand. The dimension of the exit aperture was ~7.5 mm and it was the same through the baffles, so I bought it.
Took it down to the farm, set up a target at approx 100m, loaded up a single round and fired a sighter. The good news was that I did not launch something resembling a baton round, and as it was the first time I had ever used a rifle with a moderator I was suitably impressed with both the sound reduction and the recoil reduction...
But, as put a few more rounds through it my normal quite poor grouping degenerated into quite appalling grouping, I would be lucky to hit a 6" circle consistently. My brother was with me and fired just two shots, both into the 2" bull and declared that there was nowt wrong with the set up and had I considered taking up golf?
Anyway I took the moderator off and shot an acceptable group with the next 3 shots and left it at that before I demolished what little bit of confidence I had left.