Just a thumbs up for Devon Cycle Hire http://www.devoncyclehire.co.uk/ who supplied 2 MTBS for me and the wife on my long overdue return to England. We thrashed the bikes which although tidy did not have the pricest groupsets for obvious reasons, (them being hire bikes). The bikes However turned out to be beautifully maintained by a couple that know what they are doing.
They supplied other equipment as well - sensible stuff that you would actually need.
I reckon I threw the chain off about twice over a week of being in the saddle for hours at a time and that was down to my poor shifting and not the bikes.
If you are travelling down there and need a bike to do the Granite Way or something more challenging then check these guys out.