The chance of a fax NOT getting lost in a system is tiny, its just another piece of paper that anyone could have picked up or not. Its really not a good way to communicate unless someone is looking out for the information at the other end. Especially in a large office like the one Pipex no doubt have.
You will be much better off sending a letter by special/registered mail stating you want a response in 14 days or you will take legal advice/action. You will then have to look to the small claims courts to recover your losses.
You will then be able to prove Pipex got the letter and who signed for it.