Cam, perhaps I've been lucky with mine. Since I bought it three years ago, and used it in .22 guise for most of that time, I've never had to re-zero it. I check it, sure, but it's always remained constantly on zero for those three years.
It's a two screw barrel model, and had the thin plastic washer under the black transfer port, now split.
When re-assembled the rear of the stabiliser wasn't shifting back to the recommended position and it took maybe 20 goes at repositioning the spring adjuster screw inside the stabiliser until I got it right. Not sure what difference this slightly longer rearwards movement will make? I guess it's supposed to make it recoil less.
Other than that, the valve stem was completely clean and there was just a little dust around the transfer port and inside the trigger block/stabiliser housing.
I only cleaned it as I'd taken the scope and .22 barrel off to try a .177 BSA Hornet barrel I'd recently acquired from John Bowkett and had machined to fit last week by u.k.neil. My .22 BSA Hornet barrel seems extremly accurate and I wanted to try the .177 version against my barley twist to see which is the better.
I did a test, of sorts, Saturday and the barley twist seemed to be the better of the two. But then I realised the BSA barrel may have been brand new and thought the comparison may be unfair. So I've since cleaned both so as to start another test on equal footing. The BSA was actually very dirty inside, so I guess not new after all!
I'll now re-lead both barrels and see which is more accurate.
Will give Simon's ciggy paper test a go first to check sealing is all OK.