Yesterday, My wife and I regained control of my wifes flat from a squatter. Bearing in mind it was a small 2 bedroom flat, we took out 20 bin bags of quite literally in some cases [LIFTED].
How some people can live like this, especially when its not there own property arrghhh!!!!
So now we are looking at spending a lot of money to getting the flat back upto habital status.

How are your chances of recovering cleaning costs, lost rent etc?

Effectively zero.

Sorry for you. But to answer your question, some people can live like that because they have no choice and it's a step up from their usual situation.

How long was it empty?

The previous tenant handed over the keys, without our knowledge (though he paid the rent for two more months so a small bonus there) just after we had done an inspection so he had 3 months to fester and although he left quite sharpish its the absolute mess thats been left behind.

We found some sharps boxes so we were very very careful about picking things up.

Who did the previous tennant hand over the keys to? The squater, a letting agent?
I would have thought if the previous tennat had handed over the keys tot he worng person then they'd be liable for breach of contract in some way shape or form (there was a contract?).

The previous tennant handed the keys on to the squatter and yes there was a contract.
Not that that has helped much, hes in breach of that contract but from what I have been told he has no money or assets and our chances of getting anything back are close to zero. There were a lot of letters from credit cards addressed to him. In that we want payment sort of way.