Did a Google for a recipe and his is what I come up with as a substitute for the real thing!
Making Ericaceous Potting Mix
To make a simple potting mix for ericaceous plants, begin with a base of half peat moss.
Mix in 20 percent perlite, 10 percent compost, 10 percent garden soil and 10 percent sand.
If you are concerned about the environmental impacts of using peat moss in your garden, you can use a peat substitute such as coir.
Unfortunately, when it comes to substances with a high acid content, there is no suitable substitute for peat.
Read more at Gardening Know How:
What Is Ericaceous Compost: Information And Plants For Acidic Compost https://www.gardeningknowhow.com/composting/basics/ericaceous-compost-info.htm
Put another way:
To 1 x 2gallon (ca. 9 litres) Bucket of Peat (= 32 pints)
add;
6 pints (3.41 l) Perlite
3 pints (1.7 l) home made garden compost
3 pints (1.7 l) garden soil
3 pints (1.7 l) river/sharp sand
OK that is not mathematically correct in Imperial measure, but if you work in litres, this would read 4/2/2/2/litres respectively to 10 litres of Peat.
Another potential problem; is your multi-purpose compost
peat based, reduced peat based or peat less based?
If you use peat-based multi-purpose, which will have some lime in it in lieu of peat, it might work.
If you do decide to your multi-purpose compost use 9 litres of it rather than 10 + 4/2/2/2/litres of the other ingredients this might counteract the effect of the lime content!.
Best of Luck
Edit
This might also be a way over your predicament;
https://www.mixleshomeandgarden.co.uk/products/vitax-ericaceous-feed-1l-6ef1Chempak do a similar product!