EDIT: Now £22.99 - still a good price really, though I've hit expire anyway. My 'no rush' delivery is coming today :)
Cheapest I've ever seen it... and free delivery as a shade over £20! Get it while you can... great for upgrading Technic sets without this supplied, like the 42053 Volvo EW160E which comes with pneumatic components as standard but not electronics.
[This is NOT suitable for the Winter Train - you need 4 particular parts for that which are listed below, sadly that costs more than this but you may be better off buying the passenger train as suggested below and borrowing from that at Christmas time for your display - that set can be had for around £65-£70 if you shop around, whereas the motorised parts alone will set you back around £40]