Thanks everyone, very useful!

So it's a classic 410. It has a lovely walnut left handed stock too!

Reasons I want a regulator is I notice about a 3" drop or gain at around 40/50yds from 180 to 100 bar and when both shooting quarry and also medium / long range targeting, that's just too much change for me!

Agree I probably need to clean the insides, what other recommendations would people have if I don't want to drill?