Don`t take my word completly on this, but if you payed £xx for an item and you have an invoice but had to wait for delivery and in the mean time the supplier has a price increase, it`s the amount that you originaly payed prior to increase is all you have to pay (subject to small print on invoice)and it`s the supplier that looses out.
RED21 is quite correct.
Legally the contract has been agreed, the price was quoted, you agreed to pay the price, presumably you paid for them and they agreed to supply the goods. They can not then revise their offer and demand more money.
They probably have a clause that states that they reserve the right to make additional charges etc. but you should have a case if money has already changed hands. That milesone constitues a legal and binding contract between you and the supplier.
Trading Standards would be only too keen to look into this one for you.
First off though, speak to their manager and explain the situation. If no joy then inform him (by letter is best, copy to Trading Stnds) that you will be taking it up with Trading Standards.
Also, send a copy to the dealer's head office. That always concentrates the mind...., especially the branch managers!!