In the middle of the house they are cooler on the bottom, and at the the far end of the house warm on the top and cold on the bottom.
That could be a symptom of sludge (corrosion products) in the radiator, hence the suggestion of power flushing.
The corrosion would only happen if oxygen was getting into the system.
You’re not certain that there is a sludge problem; I’d suggest disconnecting and taking one or two of the affected radiators outside and flushing them out with a hose to see what comes out and whether they are improved by the treatment.
If there is a sludge problem, I’d suggest checking the pump/OV/CF arrangement, possibly power flushing and installing a magnaclean or similar device to start cleaning the system up before you fit a new boiler.
The radiators should all get uniformly hot, if the system is balanced correctly. The furthest (least favoured) ones being cooler suggests it needs balancing AND/OR a partial blockage with sludge.
I wondered whether i could fit a booster pump half way along the loop.
I wouldn’t. One pump should be capable of dealing with this system, albeit maybe a bigger pump.
100 feet isn’t much, providing the pipes and pump have been sized correctly to ensure they are capable of providing the required flow rate.
There's a single grundfos pump on the return leg to the boiler.
IMHO, it would be better on the flow, pumping away from the boiler AND the cold feed (CF) and open vent (OV) connections. The existing arrangement would probably make it more likely that air could get drawn into the system; see above re sludge.
If the boiler has a manual-reset high-limit thermostat, I’d consider converting it to a sealed system to reduce the possibility of air being drawn in.
Where are the OV and CF connections from the feed & expansion cistern, in relation to the boiler?
Is the ‘loop’ in a floor void or ceiling void (air locks?).
Is it insulated?
Are all the rads on the ground floor?
Are there any leaks (F&E tank level drops when heating off and water fill isolated)?
Check the F&E tank is adequate (it doesn’t overflow when the system is hot).
Check the F&R connections to the boiler and thermostat location conform to the installation instructions (http://www.acwilgar.co.uk/Boiler-Manual-PDF/Potterton/Potterton_Statesman_Kitchen_Install_Guide%204.pdf ) .
The boiler stat is set to a mid-way position - i'm guessing the water is about 65 degrees.
Possibly too low; if the return temperature is regularly below 55degC, it will cause condensation in, and corrosion of, the boiler on the flue side.
The recommendation in the manual was to run the boiler near the maximum, ISTR.