Righto, I used a bolt in the master cylinder to see if it was holding pressure. Sadly no, no it wasn’t.
Pulled the master apart, and found that the bore was quite bad, it looked as though the previous owner had let the fluid drain out and dry up in the bore, leaving corrosion spots.
So, I got some 2000 grit wet/dry sandpaper, wrapped it around a 12mm fuel hose, covered it in brake fluid and reemed out the bore. It looks heaps better but only time will tell.
I need to buy some silicone grease tomorrow and re-assemble the system to see if it will hold pressure.
I have attached some before and after pics below, sorry, not the best quality.
Thanks for all the suggestions guys.
If it holds pressure great, if not, I will have to find someone who can re-sleeve the bore, then I will buy the rebuild kits as suggested by DannoXYZ, and refresh the whole system. If I go down that path though, I think I will just buy a new slave cylinder and rebuild the master.