I've been back to the Sainsbury's website. It appears they do have Silver Spoon Caster Sugar.
It's listed under 'Baking Ingredients' but not under 'Sugar' in the Tea, Coffee and Sugar' listing...
Why on earth would you use caster sugar in tea or coffee? Tea requires white lumps and coffee needs demerara.
If you put your sugar in a caster?
(That's a sugar shaker.)
The problem is the listing. The 'Food Cupboard' category has sublists like Breakfast Cereals, Tea, Coffee & Sugar, 'Herbs, Spices, Gravies & Stuffings', 'Cooking & Baking Essentials', 'Rice, Pasta & Noodles' etc.
Silly me thought that 'Tea, Coffee & Sugar' would include
all the sugar they sold in the 'All sugar' sub sublist of Tea, Coffee & Sugar.
It didn't.