What sort of a Scot are you? Start with the simple stuff before you spend a load of money on it.
Are the cables correctly adjusted? Can you take the shiney outer cover off? It is three allen head screws, no strain. If you cant, take it to shop (TITS).
Are the cables damaged, or broken? TITS, or replace them your self. New cables are big bucks, but Venhill inner replacements are about a tenner.
If the cables seem Ok, can you bend them with your finger? - if not, they are too tight, if they move too much, they are too lose.
The adjusters run along the right hand side of the tank, just below where the tank sits on the frame bar. You want a little movement, not a lot. One method is to fully tighten both adjusters, checking the valve is centered, then loosen both half a turn.
Can you move the valve when the cables are loose? If not, it may have seized. Give it a firm tap with a hammer. Still no joy? TITS. Or try and remove the EXUP valve yourself. Three allen head screws, heat and strain. If they break, TITS, or drill them out. If you get it out, clean and copper grease it, and put it all back together. Then pat yourself on the back and go to the pub for a well deserved beer and brag to your mates. It it all goes wrong, go to the pub and talk about tits.