David,

You can also try the Model Torque motors. They're available in the UK from Inter City Models: http://www.intercitymodels.com/Modeltorque.html

I don't know how good they are (never used them) but may be easier to DCC than the ones Tim was suggesting. I am not sure though and it may not be relevant to you anyway.

The odd couple of Fleischmann bits I've got that have ringfield motors will get Faulhaber motors, courtesy of sb modellbau http://www.sb-modellbau.com/. Prices are going to be more than the Modeltorque, and they may require some work to fit. But they're very, very good. The website is in German, but they do apparently understand English emails.

Steph