Not sure about tuners in the mainland - but you can get a TbT kit for your gun which includes new spring with matching guides, dry moly paste and molly grease for approx 200 bucks - basically  60 pounds sterling + delivery.

Check out https://www.airrifletuning.com - there is even a step-by-step video of the process to strip, clean fit the kit and put it back together again.  I think there is also a video on fitting new piston seals 

I've got a Prosport .177 made in 2011

It's been a tack driver forever, still is but... I installed a new mainspring myself purchased from Youngs, just watched a vid on youtube it was easy as, moly here, greese there etc. i didnt put a new seal in as didnt know how without damaging it and i shot fine so didnt want to fix what wasnt broken.

Still shoots awesome considering the bluing is long gone from the barrel shroud but it twangs.

Wondering if anyone in south island knows how to sort that out or make a guide or something, whatever it needs to quieten it up again and how much it might cost.

I put a new scope on it a fixed 10* which i love but it's still slightly canted, been trying to get it right, doing my head in so I have a look again tomorrow maybe.

Not sure yet if i want to keep it as a back up or sell it and keep using my new tx but I want it with a nice quiet shot cycle again.

Does anyone here know who does this kind of tuning?

I've long lost the new seal