Update. Use a a basic cheap scourer. It works. I dabbed one in toilet-cleaning agent.
https://www.bmstores.co.uk/products/simply-everyday-stainless-steel-scourers-8pk-368276
Had top get a professional company out, luckily they said it was just lots of soft sludge that could be broken up.
I don't want that to happen again, do you have any tips on when often to do regular maintenance and what sort of maintenance do you suggest? Gauzes over the grid for a start?