You're right Mikey-simmo. Only a court appointed bailif can enter your property without your permission and/or sieze your goods. Debt collection goons (they're not actually bailifs in reality even if they like to think they are), cannot enter your home at all without your permission, and if asked to leave your land should do so immediately, otherwise they are technically trespassing.
If they try to force their way in they are breaking and entering, and if they use any type of force, or threats against you that suddenly becomes aggrevated burglary, which carries a much higher sentence…
As for proving that you aren't the debtor they're after, it's up to them to prove that you are, or that that person is living there. If they can't produce the proof or a court order there and then, tell them to leave or you'll call the police. The police will get involved, and the goons really don't want that, as an official f*ck-off from the plod means they can't return without good cause, otherwise they could be arrested.
The sad fact is though, they are probably just trying to put the frightners on you, just in case you are the debtor, and you're trying to fool them. It's not nice, and it shouldn't be allowed to happen. The OFT is trying to regulate debt collectors, so if you find out who is harrassing you, a quick call to them might help…