I'm not an MTBer much myself, but I've heard some of the best places for it here are

Fort William: http://www.ridefortwilliam.co.uk/ (includes a world cup downhill)

Glencoe area: http://www.clachaig.com/p/v/things-t...untain-biking/ (the clachaig is a pub/hotel but it's used almost exclusively by people who are walking, climbing, skiing and biking in the area, so the atmosphere's brilliant and you can rely on their advice.)

7stanes: http://www.7stanes.gov.uk/ (7 different centers spread across the borders)

The first two are in the highlands, and the 7stanes are in the south of Scotland. From what I've heard all are pretty good, and hiring bikes shouldn't be a problem.

Hope that helps, Lynne ; )