Thanks for your replies.
This machine doesn't round up the wins like others do. It just pays out 40p or 80p wins in 10p's. Any wins in whole £1's it pays out correctly in pound coins.
I've checked the hopper opto sensor, it was a little dusty so i cleaned it, but it still spills out the 10p's. I entered in to test mode and did a coin test before and after i cleaned the opto, with each press of the button it releases one 10p at a time as it should. So this makes me think it can't be the opto sensor. Am i missing something? i'm totally baffled about this one If anyone else has some suggestions please do let me know.