PS Portal left stick stopped working up/down

Bought a PS Portal from CeX around a year ago and the left analogue stick has suddenly stopped registering up/down, although left/right still works. From what I've read this seems to be a fairly common Portal fault.

I no longer have the receipt, but the purchase was made through my CeX account. Can the store see/verify the purchase from my account, or will I need the physical receipt?

I'm assuming this should be covered by CeX's 5 year warranty as there's no physical damage. Has anyone dealt with a similar Portal fault through CeX?