Sorry if this has been posted before but I went into Tesco Milton Keynes (Kingston) and found that they are knocking 20% off paper top up vouchers. They only had o2, Virgin and TescoMobile vouchers left though. I bought a £10 Tescomobile card for £7.82 ( £10 less the VAT change = £9.78 - 20% = £7.82) They had £15 vouchers too (maybe others I didn't ask). Hope this helps someone.
Edit: ooops sorry just noticed another post on this.