Very simply ericaceous compost is lime free and suitable for acid loving plants (if you water with rain water or soft water), while multi-purpose compost MAY have materials containing lime added. It would be interesting to take the ph of both of them with a soil testing kit, and I'd bet there might not be any difference. It would be safer to use ericaceous compost for any precious plant as it should be a guarantee that an acid lover won't be harmed.

Multi purpose should have a neutral ph, Bertiefox, though whether it does or not I've never tested. Ericaceous is, as you say, acidic.