Hi Dan,
If the glow plug and check engine warning lights don't work but the oil one does it's more then likely a problem with the ECU relay.
It's a brown relay mounted above the interior fuse box. Try swapping this relay with another brown relay like the A/C one under the bonnet (beside the battery).
If it is the ignition relay your indicators will also not work when it's playing up. Just something else to try.
Cheers
Rodney