Well I didnt want to bother with sand and cement^¥^ so i just used a premade mortar mix bag that's (supposedly) idiot-proof. I used the whole bag, and had nothing leftover once I'd added the water to thicken it!

I’m guessing you’re liable?

Shit, I have no idea. Is that a thing in the UK?

¥ - my dad had a habit of buying bags of both, forgetting about them, leaving them to rot in the shed for years, rinse and repeat until you have an immovable collection of expired building materials