HMRC has rules about what constitutes proof of expenditure that includes VAT that you can reclaim. 

The level of proof required for purchases over £250 are more stringent than those for purchases under £250.

For purchases above £250 the VAT invoice must show:

• An invoice number which is unique and follows on from the number of the previous invoice

• The seller's name or trading name, and address

• The seller's VAT registration number

• The invoice date

• The tax point (this is the point at which VAT is applied which may not be the same date the invoice was created)

• The customer's name or trading name, and address

• A description of the items or services being supplied

For purchases under £250 the invoice must show:

• The seller's name and address

• The seller's VAT registration number

• The time of supply (tax point)

• A description of the goods or services

It is worth bearing this in mind in case of a VAT inspection. It is always tempting to reclaim VAT on items where you don't have a VAT invoice if you know that the supplier is VAT registered but it could land you in trouble later on.