Does it lock/unlock okay using the button on the dash? If not it is pretty common and will 100% be water ingress in to the UCH which will have blown the PCB track and no doubt the relay to. The fix is a new UCH as the relays are soldered to the PCB.
hi matey yes it locks fine from the button on the dash
New key button required then, possibly a key itself i'm afraid. I've not got any of the buttons in stock atm i'm afraid so can't offer to fix it but its not a difficult job... normally around £20.
It depends exactly how it has failed: Sometimes, the solder breaks so the button itself isn't making a connection and is an easy fix. Most times though the button simply wears out so it needs replacing and re-casing.
how do i take the key apart matey
There's a guide on youtube.
Proberbly the switch that's fallen away and a new one is an easy fix. To answer your question, NO, you can't simply replace the keys PCB as it is coded to the vehicle.
Mick