Just an update on this issue for any future readers, the class 37 above was a Lima model so to rule out whether this issue was due to a single faulty decoder or the fact that a Lima model was used with a Hornby decoder, I tried adding another Hornby 8249 decoder to a slightly newer Hornby class 47 I have. Unfortunately the exact same thing happened, the loco ran pretty ok in one direction with an acceleration setting of 9 (which gave the least shuddering), however in the opposite direction the engine stuttered again at low speed (even worse than the class 37). I also tried reversing the loco direction setting on the controller to check if it was a mechanical issue and the loco then ran fine in the direction in which it was previously shuddering, but started shuddering in the direction it had been running fine in!! This indicate a possible issue with the Hornby Select controller running in the forward direction... So, on advice above I have ordered 2 each of Bachmann, Zimo and Lenz decoders to see if they function any better and have also bought a Lenz LH100 contoller set to see if any combination of decoder and controller will resolve the issue. I had planned to buy these anyway as I only got the Hornby ones in a train set so as to explore DCC while getting back into the hobby, so no harm done, just my plan was accelerated a little :) When I test these, I'll update this post with the results for any future readers that may have a similar issue :) Stay tuned...