The most common servo's seem to be the Savox or ACE/AE XP ones. I have an AE XP one, and its very good (I think its an 1/8th servo). DS1616 Model I think.
Futaba do some cheaper servo's that are very good, and savox do a plastic gear version for about £25 I think. Can't give you any model numbers, but have a look on CML's website:
http://www.cmldistribution.co.uk
Regards,
Mark