Do I need to de-sludge my central heating system or not and inhibitor?

I have a Cenebtral heating system with with a combination boiler in my 3 bedroom flat. The system is about 15 years and is sealed.
The system has had inhibitor in the system. I put it in myself. (twice since I have had to drain the system in the past).
I have just replaced a diverter valve and would like to know what I should use to add the the system.
I have had some kettling from the boiler for a while now.
I was planning the add Sentinel's X200 de-scaler and X100 inhibitor. Have I made the right decision? What do others think? What other additives makes would be better? Do I need to add a desludger? Would the past inhibitor prevented sludge to build up?
Chris...
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