You only pay VAT if you have been charged it. If the supplier is not VAT registered then you will not have been charged VAT so therefore don't pay it. The amount still needs to be shown on your VAT return in the purchases box.
This has been debated on a previous post (Quite some time ago)
I have had two conflicting answers directly from HMRC, intitially I was told that supplies from a non VAT registered person was outside the scope of VAT and therefore not included, then after a debate on here, I phoned again to be told that if the goods would normally be chargeable or exempt, they should be included.
After being pointed in the direction of the VAT700 guide (which still isn't difinitive), I have opted to include all such purchases on the return. Fortunately they are few and far between and aren't usually a problem.
I thought that if VAT would be charged had they been VAT registered it was included on the return - if it is outside the scope of VAT ie wages etc then it isn't included (?)
I agree with Sue, it's the purchase itself not whether the supplier is VAT registered. If say you bought widgets which normally are subject to VAT but the supplier is not VAT registered the widgets don't become outside the scope of VAT, therefore, they should be included as purchases on the VAT return.