A mix of the two, say half and half, will be fine. John Innes is meant to be neutral ph, but does sometimes veer towards alkaline, so a bit of ericaceous will acidify it and lighten it up.

Thanks for the info - garden centre reckon ericaceous - RHS reckon John Innes No2 - decide to go with the mixtue 50/50 ericaceous and JI No3. Cheers

There's a lot of confusion about Acer and its 'requirement' for acid soil. Japanese Acers (like yours) do not like alkaline conditions, but that does not mean they love acid soil, they grow perfectly well in neutral soil ph. Other, large, non Japanese Acers aren't fussy about soil ph at all.