When I needed my Mk3 S200 serviced I took it to Paul at Custom Airguns in Hartlepool.
http://www.customairguns.co.uk/homeHe stripped, polished & sorted my 4 stage trigger, the gritty bolt action & gave it a full service including chrono test.
I'd highly recommend him, it's shooting like a dream now!
BTW Wastrel, I believe your S200 is a Mk2, it has the front barrel band.