i have two options for a chain device (that i've found) that are within my budget of £50 they are the dmr speedguide and the mr control MRP slalom copy ISCG link below sorry its not a fancy click on it one i don't know how to do them (please enlighten me if you feel you want to)

http://www.chainreactioncycles.com/viewproduct.asp?category=Chain+Guides&ModelID=2349

speedguide is the quality piece of kit out of the 2, even though it's nothing more than a glorified meccano kit.

speedguide is good if you have patience as it involves much faffing to get it running perfect. however once you have it sorted it'll run lovely forever.

[a href="http://www.chainreactioncycles.com/viewproduct.asp?category=Chain+Guides&ModelID=2349"target=new]TA DA[/a]
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TA DA

You can also consider a Goldtec Manacle from betd - bombproof and the newer, simpler one is about fifty quid.
They are much better in the mud than the mrp-alikes.
I've been using one of the original ones for six years, and my brother has been using the cheaper one for the last year or so, and I would say definitely it's the one to get.