Careful about rentals not awing their bikes on trails. The only one I know that does is in Spain but Anton at Rent-a-Tiger does allow his bikes on trails. Note that he has a 'branch' in Malaga which makes Portugal possible if not exactly ideal ...
But I've also had very positive feedback about Eurobiketrans and very helpful contact with them about my own 'business' - so options still open!
Regs
Simon
It seems like there is plenty of Tarmac to play on without going on the trails. The big concern with renting is the huge upfront deposit which I assume then becomes the excess should you have an accident or if the bike gets stolen .