I'd be happy with Evans, our HR monkeys told us it was Halfrauds or nothing, not sure if this is enforcable but the voucher I seem to remember had a Halfords logo on it (was a couple of years ago now).
We have a couple of months window each year but where it falls seems to vary, depending on how arsed they can be…
the justification for going with Halfords was "it lets people pop in over the weekend as they are on most high streets" when I pointed out the fact that you get piss poor service, choice and value for money unless you go for a Carrera or Broadman I was basically ignored, still I might fancy a broadman in the next couple of years…