Hi Adrian –
Is there any chance that you could post a picture of the valve gear?
The way the link in Stephensons valve gear is suspended can produce symptoms such as you describe – ie running badly or not at all in one direction. This is an issue with at least one other LBSC's designs.
It is a bit puzzling at first how to post pictures on the site, but this guide should help **LINK**
Take care, Nick (another one!)
Edited By Nick Clarke 3 on 31/08/2020 11:34:42