Thanks for the tips Andy.
I've used one so far, for a very small area, using matt black enamel, which worked extremely well, and it has cleaned up nicely in my normal white spirit thinners
Even if the entire pack of 50 only allows one use per 'brush', I'll be happy, as the current cost of 5/0 and smaller brushes, which are nowhere near as good as they were some years ago ( the quality of the bristles is not as good), will still be more expensive and, more frustrating, due to the quality, or lack of, can cause ' accuracy' problems, when the tip of the brush decides to 'fan out'.