Best additive to help circulation

[Gary80gas]

I looked at an old Worcester system boiler that keeps over heating the flow pipe and shutting down. The pump is running and pipework around the pump luke warm.
If I put cylinder on the pump moves the heat through both circuits and the rad gets hot.
Im guessing the one 2m rad on 10mm pipe is providing too much resistance for the pump to overcome., once running, you can turn hot water off and it works fine.
The old lady has all the other 6 rads off.
So whats the best additive to help with circulation I can leave in the system?

 

[ShaunCorbs]

Boiler isn't mega oversized is it ?

 

[king of pipes]

7kw is probably its lowest modulation point for the boiler so only 1 rad on wont take enough heat away from the boiler as your experiencing hotwater calling and alls good not sure chemicals will do much here but sentinal x200 and x100 in together is the norm designed to be left in the system. Cheers kop