We ordered a wood burner from a specialist company in the sales. They advised us which one to buy because we live in a semi-detached house, and told us that we would have to wait about several weeks for delivery. But when it finally arrived it didn’t fit. They’re saying it’s not their problem and there’s nothing they can do about it.
It’s not clear exactly what advice you were given or why the wood burner doesn’t fit. If the problem is a technical one which the firm should have known about you can almost certainly reject the fire as unsuitable. In that case I recommend you see a solicitor if the firm won’t budge. But if you simply measured up the fireplace wrongly it may be down to you to make the necessary adjustments to install the fire.