locking cover for water tank R reg Trident

Having meticulously prepared my VW Trident for my son's family this weekend it would appear that I did not lock the cover for the water inlet (not sure what to call it) which is annoying. Thus it has fallen off somewhere between Newmarket and Beccles.  Can anyone advise where I would find a replacement.  Are they all a standard size. Would even Halfords stock them?

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Thank you! Ordered - eventually

They all tend to be a similar size, but the internal construction can vary - I've come across 3 different types, and none are compatible with the others as far as I'm aware. So if there are no marks on the inlet or keys, it would probably be best to go somewhere that has a selection and will let you try several different types.

I have given my son both sets of keys as it is so easy to get locket out - if the one I have ordered doesn’t fit I will go with the number on the keys - thank you