The law on this seems clear at first sight: The taxable value of vouchers is the cost to the employer.
However I have read in Pay Magazine that some Revenue Inspectors assume the administration fee is part of the cost of paying the employee, rather than being a cost of teh specific benefit. This approach would lead to the employee being taxed only on the face value of the voucher.
I think IR statement of practice 6/85 muddies the waters as it says:
"The expenses which should be included in the measure of liability are those which contribute more or less directly to the benefit received by the employee".
However that SP6/85 also says that the employer must include the cost of buying the services.