A substantial percentage of call outs any manufacturer will get under guarantee turn out to be nothing to do with the product and they just can’t absorb the costs. I mention at least half a dozen common reasons they get called out where the fault turns out to be not with the washing machine in my consumer article First: Is the washing machine really faulty? and your case is one of those listed.

Although very annoying for you, it’s the person who installed it who’s at fault, and neither yourself – or Hotpoint should lose money because of it.

You would be liable to pay their charge. If it was someone else’s fault you have to take it up with them to get the money back. If by any chance they refused you’d need to take them to the small claims court. With evidence from the engineers invoice you’d be likely to win hands down and get your court fees back too. I can’t imagine this being necessary though as if the invoice says the waste pipe was incorrectly installed the installer doesn’t have a leg to stand on and will have to pay or risk going to the small claims court and losing.

If the person who installed it was someone you arranged to call it’s between you and them. If Marks & Spencer arranged it then I’d say you should complain to them and get them to refund you the money. As the consumer law stands if you paid for the service via the retailer who sent a third party your contract is with the retailer and it’s up to them to then pursue the installer for their money back.