Assuming the light is on steady and not flashing then it is safe to drive it but recommended you get it looked at soon.
You will need a fault code reader plugged in to read the code and that will/may give an indication of what is wrong, I am sure most garages can do this for you, though you can buy a small reader fairly cheaply nowadays. I am afraid I am not overly familiar with the TDCi engine, does seem odd it happened just after filling it. Might be worth goggling to see if there are any common faults that can throw up the engine warning light in these circumstances.
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