If your exup valve is totally seized (like mine was - oh and the cables were snapped too) then be prepared to remove the valve.
I found the 3 bolts that hold the valve cover in place were easy to undo but the ones that hold the actual valve into the exhaust were just rusty lumps which had been rounded off and then left to rot. I ended up grinding them off and drilling the threads out. Before I got the grinder out I tried to rotate the valve by hand, then I used a pair of grips - still no joy! The bit where the cables attach to crumbled away... Anyway I ended up taking the valve out, cutting the blade of the valve off and refitted the housing to seal the exhaust - The net result is that the engine is a bit lower on low down power - It doesn't bother me as I've not known any different. I keep looking for exup valves on fleabay but I guess they're very rare.
If anyone hasn't bothered to look at or serviced the exup valve then now is the time to do it - before it's too late.