thanks, that was my first guess but I thought there is maybe a more modern solution. But if such a mesh solution is sufficient I am fine, shall be a quick fix. Any address where to get such fine mesh?
These sort of gauze strainers are used in industrial hydraulics. If you've got a supplier close then it might be worth having a look (someone like Hansaflex ?)
If not, the unions are the same diameter (same thread actually) as the banjo bolt strainers that were fitted to Norton Commando oil tanks up until the end. If you know anyone with some used Commando parts, you might be able to find one to cut the tube off.