Reporting back after a week of use….
I diluted the sealant 2:1 with water. Consistency was a bit thicker than normal latex stuff, probably due to the lumps in it that help it seal.
Popped the tyres onto the rims with ghetto inflator (Vredestein tubeless ready tyres and old Mavic Crossland wheels) and put in the sealant via the valve using a tube and syringe. Was a bit of a pain due to the thickness of it, but got it in ok. I used 120ml per tyre to be on the safe side. Pumped up to 40 psi and did the sealant dance 😉 some sealant leaking at the bead, but not lots.
Left over night and one tyre only lost 5 psi, but the other was nearly flat. Pumped up again and went for a ride. I had to pump it up a little half way round, but no burps or other problems. Since then they’ve both stayed up at full pressure. I had a look just now and pulled out a couple of little thorns, seemed to seal up ok.
So, with dilution you get nearly 2 litres for £8! Of course YMMV.