I understand the desired differential between flow and return at each radiator is 10-12C for a standard (not combi) gas central heating system.
The important differential is that at the boiler. Older non-condensing (and some condensing) boilers were designed for an 11C differential. Modern condensing boilers have a 20C differential. I think you might be getting confused between combi (instant hot water) and condensing.
The difference in flow and return temperatures at each of my radiators is considerably lower than this. It varies between a difference of 2C (lowest) and 7C (greatest).
Measuring the temperature differential is indirectly measuring the flow rate through the rad. The smaller the differential the higher the flow rate and vice versa. If the differential is only 2C, flow rate is over five times faster than it should be. This results in greater friction in the pipes, so the pump has to work harder (higher setting) to move the water round the circuit.
1. Although all of my radiators get hot should I be balancing them to achieve the 10-12C differential at each?
2. What would be the effect I would feel of changing to the higher differential?
3. Would it be a more consistent temperature?
1. Yes
2. You may be able to turn the pump down, saving electricity
3. If you mean consistent room temperature, probably not. TRVs and a room stat with TPI control are needed for that.
Also the lockshield valve appears to be on the "higher temp" pipe on some radiators but on the "lower temp" pipe on others - is this normal or have the flow & return to some of these somehow got reversed?
Its 'normal' to have the LS valve on the return (lower temp) but it doesn't really matter. If you have TRVs on your rads there's not much you can do - unless you want to swap them over! But if you have standard wheel valves you should be able to just swap the wheel head and the LS cap and the job is done.
How to balance
Detailed instructions see
How to balance a CH system
Quick method
Check Boiler Manual for correct differential (post boiler info here if the data not available).
Remove all TRV heads and open wheel valves fully.
Shut all LS valves then open each one a quarter turn.
Let the system settle down then measure the differentials.
Open the LS valve 1/12 to 1/8th turn if the differential is too high; close the same amount if too low.
Let it settle down then recheck.
You may have to go round several times and the pump speed may need to be lowered (if possible - depends on boiler).
Don't try to achieve perfection - a few degrees either side of the target will be good enough