I agree with macc, i leave an envelope and bill, but no stamp.
If they fail to pay 2 bills, the next time round I put a bill in for the 2 lots and tell them I will put them back on my books when they have paid up.
As with the whole collect/envelopes thing, for me there is no difference with the amount owed, the money comes in at the same rate, only I dont go out pestering people at night for £10, (It does look desperate)
The Bear