Is it illegal to open other peoples mail in the UK?

I had a similar problem with debt people phoning my new phone number. It transpired that the previous owner was up to there neck in debt and done a runner. By chance I found out the number of one of there relatives and passed it on to the debt people after which I changed my number by contacting the phone company. I would advise that you write across all such letters "return to sender" and post them in the post office letter box. Hope this helps.