when you go to pay them in make sure you have a few spares in your pocket. 9 times out of 10 the bank have claimed mine to be short and yet i check them.
No don't mix them - it just means that the cashier has to sort them out and make them into bags of �5 of 10p and �5 of 5p. They then keep them in cloth bags, one for 5p and one for 10p to give out as change to other customers, and hardly ever does someone come in asking for a bag of mixed change!