The slates have been cut and have now been G4 sealed. Those should be laid next weekend and then we're onto filling. There's a trade off with the slate - in sealing you gain stain, water and frost resistance but the sealant is a little browny-yellow so the slates are now a darker colour and shiny. It should look really good.
The slates will be laid on a bed of levelled cement with SBR to stick the epoxy G4'd state down onto the bed. Then marine sealant (same as the window) will be used in the joint between the slates.
It's taken a while as the slates needed fitting and cutting (30mm thick!). Washing (5 days of drying), sealing top and sides (1d) then 3 days curing before sealing the remaining side (1d) and 3 days of curing again. If there's any water in the slate the sealant will react and you get marks like water/rings which you can't remove.
I now have a water meter and about 1kg of sodium thiosulphate to declorinate/chloromide the fill. Thiosulphate is not toxic to fish so you simply throw it in and away you go - I should need 700-800g but I will probably put the entire lot in.