Okey dokey all sorted and usual you clever lot on here have saved my butt again! Stripped the valve down and it was full of horrid black gunk. Unfortunately the gasket was in a few pieces so cut a new one from thin gasket paper. With a smear of sealant it was back together. I left it overnight before refilling the brake fluid and it seems to be leak free...so far! So once again I’m in debt for the great advice and one step nearer to actually getting away soon so hats off to you clever lot!!