I"ve recently brought a palmatum rubrum can it go into normal compost

Yes it can, though better with John Innes No. 2, depending on how big it is. As its rubrum, I assume it has coloured leaves (don't know this variety well), so it will need some sun, but will also require protection from the midday sun, say between 12 and 2, to stop the leaves from burning and crisping during the summer months.

i would really put this in ericaceous compost and perhaps a light shady place.
enjoy..

I planted an Acer in the ground about 10 years ago. It didn't like it, so I then put it in a pot with ordinary compost. It didn't like that either. Whenever you would cut back something it would then die back several inches. I then transplanted it again some 3 years ago when realising it needs to be acid soil and it looks absolutly fantastic. Infact it wont be long till I have to put it in a larger pot. So me too. Acers are acid plants and I would give them some sun but not the strongest. They also like some shelter. Even your large Maples require shelter. Yeah, you'll see the odd one in a field on it's own but they do generally like to be surrounded by other trees.

Would a 50/50 mix of ericaceous and john innes no 2 be good for re-potting an acer? Mine doesn't look so healthy this year, since potting into west+ multipurpose.

the mix should be fine, but i would go with 60/40 to the ericaceous side and also liquid feed from time to time..

I'd be inclined to stick to what Stevebuk says - I've tried that West+ stuff and its as horrible as most of the other multi purpose compost available these days. I suspect the days of being able to use multi purpose for acers are gone, having experimented myself during the spring and summer this year with 6 different potting composts (all awful except one) - unless you buy Wickes own compost, which is still peat based.

What was the exception Bamboo? Thanks for the reply 60/40 it is.

Wickes own - lovely stuff, peat based.