Here is the tank. A grubby old thing that probably leaks. I haven't leak tested it yet. It was £30 and worth a punt. It is 50" x 21" x 18" and around 300 litres.
It needs resealing, I have no idea how old it is, but the silicone is peeling away in places. I may completely dismantle it and rebuild from scratch. I haven't done either job before so it should be interesting...