DIY potting compost

I’ve made my own potting compost for years.
16 liters of sieved well rotted compost
16 liters sieved mole hill soil
8 liters of washed river sand
6 good handfuls of general purpose biological fertilizer
Mix well and away you go!

Seed sowing
I part sieved well rotted leaf mould
I part washed river sand
1 part dry coffee grounds

I haven’t bought potting compost for years. The only compost I buy is to pot up things like Camelias and Blue Berries and that not very often.

I think your seed sowing mix would probably be acid enough for ericaceous plants?