We recommend uHeat for Underfloor Heating Systems, and The Wetroom Store for all things bathrooms and wetrooms. They provide discounts for forum members.
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And grease the edges before closing.You'll need to lever it up around the edge of the cover, it'll be jammed in, most concrete covers are.
When you refit it clear all the rubbish out from where the cover sits in and off the underside and edges of the cover otherwise it won't sit flush
We recommend City Plumbing Supplies, BES, and Plumbing Superstore for all plumbing supplies.