Hello my name is Ivor.
Here are my thoughts on your problem. As you have had heating alterations to the system. The reason maybe that the water pressure is low and the system took time to stabilise once all the radiators have filled. Ther maybe also air locks in the radiators. Try bleeding them to get any air out of them.
On the boile should be a guage showing the water pressure in the system should be abot 1.5 to 2 bar. If this is lower the central heating solonoid will not operate. Under the boiler should be a small tap for you to put more water into the boiler. Once the pressure is correct the boile should start.
I hope this helps you fix the problem.
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Many thanks and kind regards