So the final update is this:
I was able to safely remove the At and replace the hammer spring which appeared to be the shock spring taken from a mk4 Ford cortina.
The rifle is now much smoother to cock and less of a twang when fired. 11.3ftlb with AA field. I decided to give the barrel a gentle polish with autosol for a smoother bore.
35 yards and AA field fit into a 10p but the most interest G thing so far is: so do webley verminpel. Even at 35 yards. Don't ask me how, I don't know how a flat head does that at 35 yards. Non of my rapids or my daystate will. I took it out on the rats tonight, using the verminpel and dropped only one shot which I pulled in a rush.
I've gone from happy to thrilled with this little rifle.
Edit: anyone buying a breaker, get extra high mounts. If not your cheeks is forced into the cheek rest a little to get correct eye relief. I removed the plastic rest and have no issues