thanks Mike for putting me on the right path.the valve is free until the bolts that hold the pulley holder in place are tightened.even finger tight is enough to cause binding.i have a look at the valve and can't see any warping,the only thing of note is a bit of wear on the back of the pulley plate [I'm sure that's not what its called] it appears that it has been rubbing against the exup valve which has matching groves.It is as if the valve is a fraction too long for its housing,but I guess it cant have grown,so feck only knows whats going on! any advice more than welcome,i'm going to have a sit down and a beer before I start loosing my temper like a big tart.