Back at the sart of our house extension project, the builder included
provisional sums in his price for plumbing and electrics. Plumbing was
about £3k which he assured us was generous.

Now, as we near the end of the project, he informs us that his plumber
will need twice this amount. What's the best way to handle this? We
are shocked and furious that this wasn't mentioned earlier. (The
plumber did visit to discuss our plans months ago, but never mentioned
this. same for the builder.)

Any advice very welcome.

David