In reply to andymac:
I invoice clients for 20p p/m when I use my bike to deliver things, and I declare a percentage of the running cost (services, inner tubes, brake blocks etc) as expenses on my self-assessment.
Buying a whole bike as an expense might be a bit much, unless its only used for work, but I don't see why you couldn't declare a percentage - or put it in the section ('capital expense'?) where pricey things like cars and computers go that you use for your work.