In reply to pork pie girl:
> b. i don't mind if it's tarmac roads .. as better than nothing
The hill simply getting on to Portland from the Causeway is a decent climb...
Have a look at the map:
http://www.streetmap.co.uk/map.srf?x=368935&y=73275&z=115&sv=36...
There are no bridleways on Portland (at least visible on the map), only footpaths. You can check this using Dorset's PROW GIS:
http://explorer.geowessex.com/
Use the 'single monitor' icon to select the base map (OS), and then the 'double monitor' icon to select layers: you want the 'countryside' layers to enable PRoWs.
You'll have to go inland to find bridleways and hills for MTB, but, having done so, there are plenty. Again; use the Dorset GIS to find them; go NW of Portland, and find the Dorset Ridgeway, for instance.
Going East takes you to the Purbeck Hills, and there's a lot of good riding around there, and, unless you're super-fit, you'll need the granny ring...
Dorset is known for its uppy-downy nature; not long climbs, but a lot of them, and fairly stiff.