Hello Simon,
If you have released a bleeder and all brakes have released, then as you say, master cylinder or servo. To eliminate the master cylinder, check to see if the piston is stcking, this would show up by excess pushrod clearance. I somehow doubt this is the problem as once it was pushed down, it probably would not return. so this then points at the servo. Have you tried operating the brakes without the engine running? If they do not stick then, try with the engine running and if they stick, then it looks like the vacumm side of the servo is at fault. A process of elimination.