A stillson type wrench will tend to compress the thin pump nut onto the thread increasing the friction.
You could try soaking with WD40 but without a set of adjustable spanners (I use an 18" and 24" set) you might be best replacing the the pump valves entirely.
If you undo the connections to the 15mm gate valve and mid position B port above you can probably spring the pipes out enabling the valves to be replaced.
You could try soaking with WD40 but without a set of adjustable spanners (I use an 18" and 24" set) you might be best replacing the the pump valves entirely.
If you undo the connections to the 15mm gate valve and mid position B port above you can probably spring the pipes out enabling the valves to be replaced.