Right quick update.
I didn't send my rifle off because I have been busy shooting it, but, last night it was stripped down, cleaned, polished, and had some mega thick tar like grease applied to the spring. All in all took the best part of an hour and about 50p worth of grease.
It has totally transformed the rifle. The recoil has been dramatically reduced and now only goes horizontally into your shoulder, there is no twang from the spring, and its a lot quieter.
I'm not knocking SFS, but TBH I really don't see the need to get it tuned now, it really is that good.