Is your cylinder a direct or in-direct??
If its a direct and youve drained down, it can lead to air blockages, which are a bugger sometimes to get rid of.
You can check what cyl it is by going into loft and if there is 1 header tank, then its a direct..............2 tanks and its a in-direct.
One feeds the heating ,the other feeds the hot water cylinder.
Pump looks like its the correct way, from the limited pictures of pipework.
Also potterton suprimas flow and return normally go up and into ceiling, i have been to many systems which there have been inverted pipe loops above boiler.....meaning that air can get trapped, depending on how its piped.
Like i said ive been into many bungalows etc where ive had to squeeze through a loft with no access hardly and while losing a stone an hr in sweat. All to find some crappy pipework with no air bleed.
Last thing does your system sound noisy?
Like many people have stated, you must have a mix between filthy partially blocked pipes and air in system.
Hope that helps a little? conventional systems are always a pain to fill and remove that last trapped air. Where as a combi you can wack the pressure up and force the problem out.
Theres many ways to get rid of air and sludge but limited ways to help without being there.
Thanx
ps- the isolation valve you talk about may be a by-pass valve, if it was linked to another pipe i.e the flow... then to the return on cylinder its a by-pass which is only suppose to be partially open to pass a limited amount of flow through if all other rad valves in the house are closed.
