Thanks all
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Did this one evening during the week.
Took the light unit out and removed the bulb holders and (after tipping most of the water away) I hairdryer'ed it out.
One word of warning to anyone else reading this; be careful with the hairdryer. It gets bloomin hot, and the circular holes for your bulbs might not end up still being circles!
The rubbers were dead dry so I Vaseline'd them up nicely
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Purely out of interest, why did Citroën only fit one rear foglight? The left hand lamp unit has a holder for the bulb (and all the electrical connections for it) but there was no bulb fitted and the hole where you poke the bulbs though still had a blanking piece of plastic over it, proving that no second rear foglight has ever been fitted to it!