We will end up with a really complicated kitchen worktop, which is L shaped, extends into a window recess with very uneven shaped sides, and has to accomodate a sink, and freestanding sink... we were thinking about slate, but the cost of doing that in one piece is way over budget, and there is no way of getting it into the house anyway. Next idea was large slate 'tiles' 20mm thick, but we don't want joins.
So we started looking at concrete type of materials, so we could 'pour' a worktop on situ. Our carpenter can build a plywood worktop using 12mm ply, and a nice oak lip to create a tray which we could then pour our finish onto, and when it hardens we could polish it. BUT concrete isn't very green, and a layer 28mm thick is going to have pretty poor resistance to flexure cracking. Could I use a limecrete mix to create a polishable worktop finish, and would it be flexible enough?

You could seal it by making it Tadelakt instead of limecrete and increase its crack resistance by using lots of fibres in the mix. You'd need to experiment a bit to make sure you could get the surface finish you want.

Personally, I'd probably consider making the worktop from 25 mm or thicker ply instead, and paint it with an epoxy seal.

Earthstone is appealing as I think our carpenter could install it, but we need 900mm deep and they only do 600mm standard - we could join them and fill the joins, but it then starts getting rather expensive... I agree that something lime based is going to be too absorbent unless sealed in some way, so I'm leaning towards glassfibre reinforced concrete, despite its environmental impact.

I have done similar in solid hardwoods (beech, maple etc), sealed with clear OSMO Polyx. The Polyx does need periodic maintenance (replenishment). However I'm not in favour of wood around an underslung sink especially when it presents end-grain. An issue to be addressed with deep (wide) wood tops is that movement has to be allowed for in fixing and edge detailing.

'Earthstone', despite the beguiling name, doesn't seem very environmental.

Regarding concrete, there is no perfect circumstance, and you might just think about offsetting its use elsewhere. If your concrete formwork had an oak trim to be left in situ, the joint detail where it abuts the concrete would be a weakness. I would form a radiused concrete edge and polish along with the top. I would both reinforce the concrete and up the thickness maybe to 45mm.