Thanks to Skarlouie (a.k.a. Skarloey on the bachmann board and world of thomas modelling), the finished rework of Arthur is now completed!
The buffer beam was modified three times to get it looking right once and for all, and with the new plate fully finished, the model looks a lot better and far more accurate in detail. I compared the model to images around the web, even screenshots of the TV series 'Arthur', and there is no doubt in my mind that he is based on a class 3MT 2-6-2T.
The Ivatt Class 2MT 2-6-2T is smaller than the Standard Class 3MT, so there is no way that the tv series Arthur is based on this class of engine.. Otherwise, it would mean that Thomas is the same size, which would be wrong.
However, there is nothing to say that couldn't be a Ivatt Class 2MT, but photos around the web, looking at both versions of the locomotive, id say my money is on the 3MT. It's also possible that Arthur was built as a 'Hybrid' locomotive.