What causes the high levels of soot/carbon build up in the egr/inlet???

Most commonly it's mainly only doing short distance driving that causes it. Higher speed/longer runs are needed to clear everything out as the temperatures get higher. Short journeys only and you don't reach the required temp for cleaning it out and the soot builds up.

I know that you have taken your EGR valve off recently to clean it dave, my advice would be that you take it off again and clean it but do this aswell.
Once you have it clean put a little oil on the valve stem and work the valve from the exhaust gas side, i cleaned mine to the point that i could see my face in it but after 1.5 tins of carb cleaner the valve was still very stiff to the point that it would stick if i fully depressed it so i lubed it up and just kept working the valve with a wooden dowel.

This sounds stupid but do you mean internally ie in the indentation on the inside of egr ??

Thats the one, you can see the metal stem of the valve coming off the diaphragm. Lubricate this and work it, it will most likely be stiff for the start but it will become loose and springy like it should be.