First off, don't worry, they are not your debts.
Phoning the companies may not do any good, because lets face it, woul you believe that?
Best to write, explain the situation, enclose a copy of a utility bill as proof, and ask them to stop.
Do NOT marked letters not known and put them back in the post, as they may not get there, and if they do it will take time.
At home, make sure you can lay your hands quickly on proof of ID, utility bills, in fact anything that will back up your claim not to be the person they are chasing. This is in case baliffs come, and if they do, like I say, don't worry because it is not your debt. Any problems with baliffs (there shouldn't be) simply call the police and let them deal with. Oh, by problems, I mean not believing you and going away, not forcing entry or anything.
I once had a situation where a baliff wasn't accepting what I said and thought I was trying it on. Then I caught a look at his paperwork which I noticed had a different address, so I called the police instead. He thought as I had done it anyway, the copper would just back him up that he had certain rights, but boy was his face red when he found out the truth!