Basically the boiler is reaching temp to quick because it can't get rid of its heat now because the right hand pipe is getting hot for about two mtrs it lookls like the water isn't moving you need to check the pump because they have got a habit of throwing the impeller so it sounds like the pump is running but it's not also check the impeller isn't blocked with rust or debris. Plus one THING TO CHECK MOST IMPORTANT ON OLD GRAVITY SYSTEMS PLUMBERS WOULD FIT A NONE RETURN VALV ON THE PIPE WORK . YOU NEED TO FID IT AND REMOVE IT BECAUS IT WILL NEVER WORK IF ITS ON THE WRONG SIDE. HOPE THIS HELPS PAUL .GAS SAFE ENGINEER