In reply to FinrodFelagund:
I'll get flamed for this, but the simple answer is "Yes".
A slightly better answer is akin to Fausto Coppi - "Only when absolutely necessary"
As you say Cycling on a footpath is not illegal, but it is an act of trespass and the landowner can insist you leave. So you are in exactly the same position as Benny Rothman and his pals on Kinder in the 1930s. And for exactly the same reason - access laws have not kept up to date with the legitimate desire for recreation, and we are still trapped by the notions of property ownership that date back to 1066.
The law that grants cyclists access to bridleways dates back to 1968, a good 10-15 years before mountain bikes started to drift over here from California. Cyclists may be tolerated on bridleways, but their requirements are not considered. So just as with roads we are secondary to other modes of transport.
Does this mean I think MTBs should have unfettered access - absolutely not. Whilst my respect for the law may be limited, my respect for the environment and for other users is paramount.
So before you ride an FP, consider whether it is likely to be regarded as anti-social or a waste of goodwill. Avoid it when the ground is soft and tyre tracks are very obvious. Avoid it when the trails are busy with walkers. Don't ride in big groups. Keep your speed down unless you are absolutely sure you can see the trail ahead. If you do meet walkers, be exceptionally nice and friendly, even if they are not.
Mind you, I would advise following these rules on BWs as well.
Post edited at 16:31