Can you explain exactly where this "DHW cap" was? Was it underneath or on the flue pipe on top?
I can assure you that grey sand is not something that should be added to your boiler! What you may be looking at is a build up of limescale in the pipes or contamination in the flue.
My personal feeling is that, simply by asking if you should add sand to your boiler, you are in grave danger of attempting a repair that could put you and your family in danger. Please call a qualified Gas Safe engineer to perform the correct repair and give your boiler it's long overdue service.
Rupert
June 2020
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