It's a fairly common Mk5 problem. I had it on the passenger side front door on my previous GTi. It wouldn't open for days, however the night before it was due to be looked at by the local dealer it re-opened. The dealer couldn't replicated the fault and gave my wife a patronising lecture on operating the central locking...... However, they didn't bother to run a diagnostic on it, and when I subsequently had it serviced at an independent they found two 'central locking convenience module' fault codes logged when undertaking a VAGCOM scan! However, the fault never came back again, and it was probably caused by my son pulling on the door handle whilst the car was being unlocked.
Hopefully, you can get it open with the key, and it won't re-occur again.
