I phoned my local council about this recently and was told that you should remove any plastic from cardboard boxes. You may already know this, but you should also try to remove the windows from envelopes or just throw them away with the unrecyclable rubbish and always remove the lids from plastic bottles.
i just put all cardboard and paper in 1 bag, cans in another and plastic, i dont bother crushing the cans/plastic and i keep the lids on my plastic bottles and they still get taken away.
THFCUK - just cos they get taken away doesn't mean the lids can be recycled. It just means some por sod has to go through the batch of bottles taking off all the lids that people couldn't be bothered to remove themselves.