Thank you very much jcr, that's a really good article. Unfortunately, I'd already taken most of the steps explained there. I ended up with a pulley wheel that was so corroded it broke up, lots of snapped bolts and an exup valve that was seized so tight in the housing that I think someone must've welded it there for a laugh.
I managed to break the exup blade trying to get the damn thing out. To replace all the parts was in excess of £700, versus 294 for a Black Widow system.
I'd recommend the Black Widow solution very highly. Beautifully made, perfect fit, brilliant delivery and the bike sounds awesome now. Absolute bargain and no more exup misery.