My first step would be to turn off all rads except one (say nearest to boiler) and see what happens to that rad. I had a similar problem on my sister's system I also worked on as a favour. It is a completely different layout but the cause was a dodgy motorised valve to the heating circuit was getting very little flow. In your case could be the pump is buggered. Also, are you able to adjust the speed on your pump?

If the test shows a perfectly functioning rad, you can then start switching others on one by one. Don't turn them on full though just so they are getting hot.


WRT to the lockshield question other than convenience when removing the rad, given it is a single pipe system, it shouldn't impact the function of the heating system as the valve the other end will be able to reduce the flow through the rad to balance the system.