See the mzv,is the pipe from the pump going up and then to your boiler,and the other 2 one to cylinder and 1 to rads,a s its fully pumped,just a strange way to pipe up,the valve like that,so how is the pipe above cold and below the pump is warm,so its on the return,im just working it out,ok now if its warm below the pump,then see the pump valves,make sure u close them,then try and undo the big nut on the pump,and then open the valve above and see if theres water,or if the valves has sheared off,have u been in the loft and seen the level of water in small tank,if u have and its full,then make sure no valves r closed on c/feed small 15mm u can see on right of pump,trace it back up and see if a valve is closed