Folowing on from this thread....
I thought there might be more people able to help (now that everyone's gone back to work after the BH weekend)!
Basically, replaced Sora 50t ring with a Stronlight 48t ring and can't get it to shift from outer to inner (34t) for love nor money. It will shift from inner to outer with no issue
I altered the mech/ring spacing (position on seat tube) to accommodate the (very slightly) smaller ring, re-tensioned the cable, checked the mech angle and checked the limit screws.
But still couldn't get it to shift.
I put the original 50t ring back on and had it shifting fine within about 5 mins.
One thing/difference I have noticed is that the Sora ring is dished/offset slightly so on a flat surface with the spider arms down, the teeth are lifted off the surface slightly. The stronglight ring is completely flat. I wonder if is this lack of offset between the spider and the teeth is causing the problem?
Another thing that makes me think the offset might be something to do with it, is that with the Stonglight ring fitted and the chain in the inner ring, when shifting down the cassette to the smaller sprockets, the chain scrapes against the inner face of the outer chainring.
It doesn't do this with the original Sora ring (making me thing that the offset moves the ring slightly further outboard so the chain misses).
I did wonder if I have got the ring the wrong way round, but don't think that I can have. There are shallow counterbores on one side for the chainring bolts that if I turned the ring around would sit against the crank spider and wouldn't be doing anything useful.