I was asked to talk to the manager at my part-time workplace which is a supermarket.?

If you are innocent you should have nothing to fear. They are clearly incompetent managers if it takes them weeks to notice a £5 error. It should be obvious within 24 hours.

Be sure you understand the procedures for any cash transactions, and if you don't put it on record that you requested training (they can't accuse you of errors if you were not trained).

It will probably blow over, but unfortunately supermarkets have to have a 'zero tolerance' approach to everything (like not drinking from a dented can, or eating a bruised apple) because they fear theft and fraud on a large scale. [Personally I think that if they treated employees better the problem would solve itself, but that's another story].

If they decide there is a disciplinary case to answer you must be formally notified and have a right to be accompanied. You can check the process from the ACAS Code and Guidelines.

I used to work as a supervisor in a clothes retail shop and I was responsible for balancing the tills at the end of each day. The company allowed for a £5 discrepancy per till either over or below as this was a buffer for giving wrong change on occasions.

Basically if the discrepancy was more than £5 we would just investigate every member of staff that had used that till and ask if they had taken the money or believed they may have given incorrect change. Just a quick bit of paper work and then we would send that off to our head office so I could balance the float again. Pretty much all of the employees would say they don't remember giving wrong change and they didn't steal it. Occasionally a staff member would say they did give the wrong change but they were never punished or fired unless it was a regular thing.

Seems to me that it's standard practice for your company to look into it but they've left it very late, as were I use to work the investigation was carried out the following day.

Just be honest, and if you can't remember wrongly charging a customer then just say that. You probably haven't been investigated so far as your shop as a whole wasn't that bothered, it's probably the head office that is on the case of the manager to check everything.

This all sounds a bit strange to me..... if there is no record on the till receipt, then how do they know you were asked to carry out the transaction? where is the evidence?

Secondly, they are not claiming that the till was short so where is there a case for any action against you.

If they are going to accuse you of anything dishonest the onus is on your employers to come up with some solid evidence and backed up by witnesses. Be firm with them and ask for their evidence, otherwise I would submit a grievance on the grounds of defamation of character