Allan,
My first thought, other than the obvious "is your water separator full or damaged?" is, how long is your hose from the separator to the airbrush? I found that when it was very humid out, or quite cool sometimes, I still got a bit of moisture in the 10' braided hose I use. If that seems to be the case, may I recommend one of the small filters you can add right at the hose/aibrush connection.
http://www.harborfreight.com/airbrush-mini-filter-96427.html
I know you're in the UK, so probably don't have harborfreight tools, but I incuded the link as a visual example. I don't use mine most of the time, but when it is very hot and humid, I find I need it.
Hope this helps, and welcome to the forums!
Chris
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