Yes, it leaks from there when we re-pressurise. Also there is a brown streak down the paintwork from the discharge pipe, so it must have been leaking for some time. The system has been losing pressure like this for a few months.
What is the advantage of a pressurised system over an unpressurised system? It feels lots more elements that can break and need maintaining. We had a gravity feed with a couple of pumps before and it worked perfectly. The only reason for pressurising the system was to avoid the cost and hassle of a power flush. Was that a false economy?