I had a look around to see which motor I wanted to use, and decided to give these a try. They are supplied by Conrad Electronics, a German electronics retailer similar to Maplin in the UK. They also have a UK webshop, and you can order these motors in packs of 3 for very cheap. Clicking here will take you directly to the corresponding page. These are motorised drives, so not coil driven like a SEEP. However they are not slowmotion drives, these switch within 0.5 seconds, but switch off at the end of the throw so you don't get any 'sound effects' such as hum etc.
They also do not have an extra switch to control the polarity of the frog, however the actuating lever is protruding at both sides of the housing, allowing for left or right hand side mounting. The unused side could easily be used to actuate a micro switch installed next to the drive, for that purpose, and that's what I am planning to do in due course.
I don't know much about the reliability of these, so time will tell, but for the money I thought it was worth a try.
To see one in action, see the below video. It is driven here by a programmable DCC accessory decoder, which has the advantage of setting the power to the motor, slowing it down a bit. I intend to do the same.