Coed-y-Brenin hire out bikes, no prices on website...
http://www.beicsbrenin.co.uk/
R.H.Roberts bike shop in Bala also hires bikes out, he had a nice full sus Scott in the window on Saturday - 01678 520252 but a bit out your way if coming from Brum to Mach (depends on which BH we stay at.)
A google shows this bike shop in Mach does hire, but not listed on their website
http://www.theholeytrail.co.uk/index.php
This fella also hires bikes, just down the road from Coed-y-Brenin
http://www.dolgellaucycles.co.uk/
EDIT:
This might be just what you are looking for....
http://www.mrx-bikes.co.uk/
HTH