Well I actually did it the old skool way in the end. Drained down system, discovered blocked feed tee and re did. Flushed through, wacked some fernox f5 in for a couple od days, returned and drained and then took all rads outside and flushed with the mains. No leaks. And all rads are roasting. Some of them I had to flush for around 20minutes. Was so exciting🙂