Had this happen recently with UB and its quite frustrating,I had to go to an UB branch that Monday and one of the counter clerks took all my details and told me it would be sent upwards and be investigated and the money refunded asap if the machine didn't balance.
I got the money next the next day.
That security crowd need to be reported,they should not have attempted to refill the machine when customers were still using it their attitude further to that is unacceptable and you should report the matter cc both BOI customer complaints and G4S security on this matter.