There's a whole host of info on HMRC's website about business promotion schemes and Face Value Vouchers ("FVV") like this, but I wouldn't go reading through all of it.
The upshot is that the tax treatment of face value vouchers depends on whether you are redeeming it from the same person who issued it (retailer vouchers) or some other type of FVV. Given that it is a supplier discount from one of your suppliers, I'll assume that it is a retailer voucher.
The tax treatment of the FVV is the responsibility of the supplier when the voucher is redeemed. Effectively, the supplier will treat this as a form of discount to you for your purchases. The normal rules for discounts then apply with VAT being due on the discounted amount.
The supplier should account for the VAT in their systems, reflecting the FVV as a discount and giving you a tax receipt or invoice showing the discounted NET value of the goods with VAT then calculated at the applicable rate on top.
This receipt or invoice should then be no different from any others that you have and you should be able to post it straight to your system without any further messing around.
I hope this helps
C