Sorry, its a bit more complicated than that, I wasn't looking at the picture properly in the first place earlier. (Joys of a 2 year old who's just found the hammer I'd forgotten was in the living room windowsill....) In your situation the Pump would need moving I think, as you need to create a similar arrangement to that in the picture you posted earlier of your Father in Law's system.
The Flow pipe from the boiler needs to rise continuously with no restrictions to a suitable point over the F&E Cistern. (Have to wonder if that's really a safe setup you have currently if the pump is in the run between the boiler and open vent...) Then at a suitable/convenient point, Tee off, and couple the cold feed in within 150mm of the vent, to create your Neutral point. From there run to the pump, then back down to your 3 port valve.
If there is room to rotate the 3 port valve by 90° then the job should be relatively straightforward. Remove pump, connect pipes across former pump position. Further up, cut in a Tee, go to the right slightly, connect cold feed, then turn down 90°. Fit pump here, then run down to 3 port Valve, turning towards cylinder 90° to enter valve. Cap off former cold feed as close to point of connection as possible.