I think there's a difference between rubbish being left and the train being 'dirty'. I've also seen many FCC staff (or contractors) going through during the turnaround with a bin bag collecting papers, bottles, cartons etc - and other passengers who have already boarded usually help by passing things over.
At night, I guess FCC don't employ them. The train I saw at King's Cross Friday night looked like a group of people had raided an off licence.. although, to be fair, the bottles and cans were all placed neatly on the (small) tables. I'm sure the bins on that train were probably full.
When it comes to actual dirt, I find the FCC trains to be very clean indeed - and it's clear they get cleaned regularly. I've never got on a train in the morning to find it messy. I also think they must clean the seat bases quite often too, as I've seen the mess left by shoes on seats - but that would mean they'd all be ruined by now if they weren't cleaned.
What's more, there's a lady that I see very often (so she must work almost 24/7!!) walking through trains and inspecting seat backs, dust on the top of route information signs, behind seats, toilets etc - and noting it all down. This suggests FCC is constantly checking that their cleaners are doing a good job.
I can't speak for other operators as I don't use them often enough, but overall I'd say their trains are kept in excellent condition - considering it only takes one single trip (possibly only a few stops) for passengers to fill it with rubbish.
