You basically get an interest free loan and repay by instalments. The instalment you repay each month is not subject to Tax or NI so you benefit again. There is also the potential to reclaim the VAT on the bike if your employer claims VAT normally.
Only thing to check is impact on pension, plus at the end of you repaying all the instalments you still have to buy the bike off your employer at the going value of the bike. There is no set rule how much this is but ~5% of purchase price plus VAT if your employer can not reclaim the VAT.
http://www.cyclescheme.co.uk/
btw Martin, how's your swimming?
I've just decided I don't trust you anymore.