Can you just dump the slummy in a hopper or do you have to feed it in coin by coin ?
(I never use the things)
The Sainsbury's ones I use for the purpose have a slot that will accept 3-4 coins at a time, and some sort of buffer that it works its way through. You can, with a bit of practice, pour coins into the slot from your hand, but if you overflow the buffer they immediately reappear in the change hopper. Once you get the hang of it, you can hear the mechanical click that means it's ready to accept some more coins. Eventually it will fill up and refuse to accept a particular denomination.
My general approach is that whenever I have to buy some milk or whatever, I fill a pocket with random change, feed that to the machine, and (much kudos to the designer of the machine for allowing it) top up with card payment to complete the purchase.
(Top tip: If you've just fed it a zillion 2p coins, and you then try to pay the remainder with a larger denomination coin, it will give you the change in 2p coins, because for some reason it has loads of the fuckers.)