Hi welcome to GoY, Japanese Acers prefer a slightly acidic soil, so mixing the ashes which are alkaline, into ericaceous compost, is not really a good idea, they will grow in other soils but will not tolerate, wet, dry or very alkaline soils, I think it would be better to spread the ashes all over your garden very lightly, Derek.

You could bury them in the garden and plant a more tolerant plant over them though. Or lots of the smaller roses will be happy in containers and would probably be OK and more tolerant than acers which need a half shaded sheltered area.

If you really want an acer you could put the ashes in a container and bury that, in which case the ash would not affect the acidity of the compost.

Thanks for the advice. I spread the ashes in a corner of the garden this morning. Keith