Thank you everyone. My last question is, to get the best deal, should I buy now, after Boxing Day or wait until February for the voucher?
As I posted above - both.
Ask the retailer if you can buy now and use the voucher later. Evans do this, plus you can buy a sale bike without an 'admin fee' that almost all other retailers will charge. I just got a 2015 bike, negotiated a discount and will be back with a voucher for a partial refund in the new year.