HorT, having looked at this in my workplace there are various ways of doing it but getting it in place will require some careful negoiation with mangement around such issues such as who foots the bill for the bike (employer or a cost neutral method via 3rd party) and how easy it is to add a salary sacrifice deduction to employees in the payroll software and then to monitor the deductions taken. Sure it is easy to do this for one or two employees but monitoring hundreds can become a headache, especially trying to keep track of the deductions made.
You also need to consider wider issues such as will the be benefit be availble to contract staff. RE: 12 week agency worker regulations, what does the company need to provide in way of changing and washing facilities, lock ups etc.
To summarise, getting it up and running is easy peasy it is the other bits of red tape that need resolving which can often lead to employers dragging their heels.