Just a quick one (couldn't find if anyone else had asked this question already, so apologies if I'm repeating a thread!)

I need to make a large batch of SMBC this weekend, and I was wondering if anyone has any experience in making it using margarine/baking butter etc. (in, say, a 50/50 mix with regular butter) instead of just regular butter? I want to know if this is a good/bad idea before I buy ingredients or potentially mess up a whole batch

Bad idea! It's meant to be made with butter.

Don't even make it with cheap butter. You are just buying water, no real savings and the quality will suffer.