I left the silver nitrate in the tank for quite a while and it leaked.
I was down to about 300 ml of solution and quite a bit of discoloured crystalised nitrate in the bottom of the tray I luckily had my tank in.
I tried some plates in the nitrate I made up to a litre with distilled water, to see what would happen.
Quite predictable, didn’t work.
The hydrometer sank to the bottom when I tried to measure the gravity.
I decided that the rescuing the crystals in the bottom of the tray was not a good idea as I feel it wouldn’t give reliable results, so I ordered another 100g of Silver Nitrate from here.
I added the nitrate to the weak solution until the gravity reached 1.067., resealed the tank with superglue and I’m now back in business.
Lesson learned, store the Silver Nitrate in a suitable and clearly labelled sealed bottle when not in use.