The Sale of Goods Act gives you rights when you buy goods from a trader.
The item must be (amongst others) fit for the purpose and of suitable quality - meaning they should be reasonably durable.
The consumer rights last for 6 years from the date of purchase.
A �300 fire should reasonably last longer than 18 months.
Your contract is with B&Q - you bought the fire from them, so you should pursue your claim against them.
It is too late to claim a full refund - but you can claim repair, an exchange or a refund of part of the price.
The fact you no longer have a receipt can be dealt with. If you paid by credit card, debit card or cheque you will be able to get a statement showing the transaction. (You may have to pay). If you bought it on finance you can produce the paperwork to show proof of purchase.
If B&Q refuse to remedy the situation, do not hesitate to take it to the small claims court. It is easier than you think, and cheaper than replacing your fire or bearing the cost of repair yourself.