Begonias will do well in dappled shade. They come in every color, trailing or upright. Many are prized just for the outstanding foliage such Rex Begoinia. Also look at Coleus.

Thankyou! I’ll look into them! The planters are going behind a border with tallish (3 foot tall hydrangea in) so I’m thinking of ynderolabting the acers in the pots so that the brim of the pots is less visible from the house!

Remember that Bath's suggestions will be summer only as they are both tender. Finding this a bit hard to visualise - the hydrangeas will quickly get much taller than that unless they too are in pots too won't they. Were you thinking of something creeping that will disguise the rims in winter or something taller that will show up in summer when the hydrangeas are in full leaf? Won't underplanting compete with the acers? You might think about using large pebbles instead?

I'm thinking Heucheras, Lamium maculatum, Vinca minor, maybe Pulmonarias.

I underplanted mine with red house leeks and it worked beautifully giving me winter interest after the leaves on the Acer had fallen. I can’t show you as I put the Acer in the ground in the autumn.

Thanks everybody there’s a lot to think about about now, the pots themselves are about 4.5 foot tall and a good 3 foot wide and the hydrangea infront of one is in the ground whereas the pots will be elevated about 10 Cm as they’re going to be on a deck behind Nd the border, so hoping overall they’ll add some height :)

Ah, well you might like trailers then Dan. And I have one that would be nice that I discovered this year. Pratia. You wouldn't want it in your garden as it spreads everywhere, but it looks lovely in a pot trailing over the edges like a waterfall. And you can control it there more easily! This is the evergreen, white flowered one I use in my pots....http://www.bethchatto.co.uk/o-r/pratia/pratia-angulata-treadwellii.htm