In the UK is a pro-forma invoice an offer of a contract?

I recently asked for an invoice for a yearly contract for my dedicated server. The invoice arrived by email as a pro-forma which I duly paid (Spanish bank account where I live as an ex-pat). The company then told me that they had made a mistake and they had given me the wrong figures. Here are the original figures:
Amount Due: 拢1,702.13
VAT: 拢297.87
Total: 拢2,000.00
No vat applies here so I paid (as is correct) 1,702.13
They said they had meant to charge me as follows:
Amount Due:拢2,000.00
VAT: 拢350.00
Total: 拢2,350.00
So I should have paid 拢2000.00.
I replied that I had budgeted the amount on the pro-forma which was an offer. I paid the 1,702 in good faith and so accepted the offer and a contract was made.
I have a feeling that contract law is on my side here. Am I correct? Please only answer if you know if know if the law of contract is on my side. They have admitted it was their mistake.

A proforma invoice is not a contract. It is more of a request of payment before goods/services are provided/released.

If the invoice stated E &O E, then you will have difficulty challening it as those initials stand for Errors & Ommissions Excepted.

However if hey gave you a fixed price last year and didn't mention a hike in the service charge then you may be able to appeal on this basis.

since the payment was already effected according to their quote (which is what a proforma invoice is really and truly) they should honor their first offer. also, they admit it was their mistake. however you should check the proforma invoice and see if there are any fine prints, and also if they gave any validity of offer. i think you have a valid point.

A pro forma invoice is a request for payment. The CONTRACT was agreed beforehand. If no price was agreed during negotiations, the amount demanded ( and accepted) on the pro forma is the contract price.
Did they know you weren't UK based and not liable for VAT? I suspect they did, which is why they issued the original invoice for 拢2000 inclusive in error.