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Thank you very much for your question.
The glow plug warning light will flash or illuminate when the engine management unit (ECU) detects a fault. This fault will have been stored in the ECU's memory and can be retrieved using a suitable code reader. When the light is on or flashing, the engine may be put into a safety mode to prevent any possible damage occurring, if this happens, you might feel a drop in performance.
You will need to have a diagnostic check carried out to determine exactly what the fault is.
Most garages are equipped to carry out this check nowadays, so I would suggest phoning around for a quote unless you use a garage regularly. You do not have to go to a main dealer. The most common faults are faulty cam or crank sensors, but many other things can cause the light to flash, so guessing isn't really a viable option and will certainly cost you money.
Continuing to drive your car should not be a problem if you take it easy. However my recommendation would be to have a diagnostic check carried out as soon as possible, just in case.
I hope this answers your question.