Thanks very much chaps.
Aaron, I know what you mean, but there was no sign of water, the airbrush had been thoroughly dried and blown through, and the Future/Flat Base mixed as normal. It's not possible to strain the Flat Base, as it's a grey, thick substance, rather like glue. But it dissolves and mixes without problem.
The Future as a gloss coat was applied by brush, and allowed to set for over 24 hours,by which time it was hard, and scratch-resistant, before spraying the matt coat.
I think the problem is with the Future. This bottle has been used before, for exactly the same processes, without problems, although the gloss finish was nowhere near as good as the 'old' stuff. I think it may have 'gone off' - OK for floor polish, but no good for modelling. The visible yellowish-brown tinge has not been visible before, and the change to a 'solid' , white bottle , from the original clear bottle, might be an indication that it could be ,for example, light sensitive.
It certainly didn't give the smooth, glass-like finish, when brushed, that I'm used to, and I've taken the decision not to use it again, apart from maybe the odd 'gloss only' use, on small areas.
Andy, I brushed on the Tamiya clear gloss (rather heavy viscosity), and was quite surprised it was dry within an hour, and solid as rock 18 hours later. No doubt the clear matt, when thinned and sprayed, will take some time to dry though.
Not got any more done yet, as I've felt as rough as a bear's Rs today, with the beginnings of yet another cold - Mick's fault! Hope to get some done tomorrow though.
Thanks again for all the positive comments and encouragement.