[Neanderman]

Did you guys make a concave structure that remained with the sink <snip> did you build a sink out of wood and line it with fibreglass, leaving the inside/working surface as raw GRP then install the whole thing

This was my approach (though I'm not sure what you mean by "raw GRP"...

Did you build an anti-sink out of wood, line it with fibreglass, pop the wooden plug out and then just install the fibreglass shell?

This could be one approach, especially if you were at all interested in making more than one (or if, as you indicate, you know someone who does this kind or work.) But for a one off, I think in the end it's a lot more work.

Mine is really quite basic -- all of the internal corners are square. I drilled the hole for the drain before I glassed it to minimize swelling of the wood due to any leaks around the drain, (which is a non-type 316 stainless steel insert purchased at a home center, so it developed some rust spots pretty quickly) and I think I only used one layer of fiberglass cloth. At the most, I used two. I did put down multiple coats of resin, so I'd have as smooth a surface as possible. The resin was spread using a rigid plastic 'squeegee' of sorts that you can get at the auto parts store.

Once the resin has hardened and is thick enough for your tastes, having a sander to smooth it out is helpful. I wish I'd had my random-orbit sander back then as I think I could have made it even nicer.

I occasionally think about reworking it with some more fiberglass, but it's a big job and the sink is functional as it is, so I live with the little defects. Fiberglassing is an art and I am no master.