2009 Caddy in safe mode, won't start and glow plug light flashing

[PRMG]

My 59 reg Caddy 1.9tdi won't start.
I drove it home from work with no issues whatsoever, I went to go back out 10 minutes later and it cuts out after a few seconds, the glow plug light is flashing and SAFE MODE is displayed on the instrument cluster.
When I got in it at work the remote central locking worked fine, since getting it home it isn't and I have to put the key in the door.
I have OBDII & Torque Pro, but after 95% it comes up no fault codes found.
Can anyone help please?

[Doc]

SAFE on your instrument cluster means the key transponder is not found or recognised.
Do you have a spare?

[PRMG]

I tried the other key and all is good, thank you.
Why could this happen to the other key? I believe they are both the original keys, will it just need re-coding?

[Doc]

The immobiliser transponder (glass pill) in the key might have broken if the key was let fall. Some aftermarket keys can separate and the pill can just fall out. Either way, your lucky you have a spare and not had to have VW order you a new key. In a "lost all keys" situation, they charge up to £400 for a replacement.

[PRMG]

Thanks, I'll open it up when I'm home.
Only use for it atm is to unlock the doors!
Very relieved that was the problem though