Hiya Ratts,
As you say, proper silver sand mixed with white cement. I've used it for external rendering. No painting needed, the finish is just what you're looking for.
Here in Devon, although I haven't used it for a long time, it came up from Cornwall. (Granite country.)
There's a lot of granite in Wales. (Assuming you're still there.) You really need to see the product for yourself, so maybe check to see if there are any quarries near you.
I know the yellow sand. It's probably "pit sand", dug up rather than the result of crushed stone from a quarry.
Look out for any granite quarries in your area. Sorry I can't be of any help.