The overall width of an 8 and a 10 speed cassette are very very similar. I would look elsewhere other than the derailleur itself.
10 speed is 37.2 mm and 8 speed 35.4 so only 1.8mm difference. (the width of an 8 speed sprocket).
Cheers - the extra distance I need is more like 10mm so I maybe it's just the frame.
So (theoretically at least) the old mech should pretty much cover the distance of the new cassette?
The overall width of an 8 and a 10 speed cassette are very very similar. I would look elsewhere other than the derailleur itself.
10 speed is 37.2 mm and 8 speed 35.4 so only 1.8mm difference. (the width of an 8 speed sprocket).