Alternative to natural slate

I would use natural slate. The cost difference for a small area isn't significant. You can save by going thinner.

Actually, I may have remembered that exactly the wrong way round. There are price differences in different thicknesses but another source says that the higher quality are thinner (as they're stronger and lighter).

Anyway, worth investigating.

IMO natural slate is exceptionally good looking and durable and so not worth cutting corners on for a relatively small area.

I was going to suggest Killoran slate company but their website suggests they are closed and to contact http://www.bluebangor.ie/

I once sourced some s/h blue bangors from them to match and repair a building I owned whose roof had been there since 1893.