Two questions!

[ArseyTussle]

How do I prune a buddleia to maximise density of growth and flowers, and stop it getting leggy and woody?

Also, what would look nice at the base of an Amelanchier? The bed is only 1m square, so I'm wondering whether some blue green low evergreen grasses are the way to go, or just ground cover. Or something else?

I prune my buddleia according to age/size of bush. The large old one gets pruned to a couple of buds before the old trunk. The newer smaller one gets pruned down to about 20 cms above the ground.

At the base of the Amelanchier, I'd put in some spring bulbs, for interest when it's dormant and then something like cranesbill geranium with a large blue/ purple flower, ( there are several varieties). They would contrast when the tree is in blossom and be easy to maintain.

I would not put grasses under the Alamanchier as they will be in competition. Summer bulbs, blue bells, crocus, snowdrops , fritillary and forget -me not would work. My personal opinion though is to just put bark chippings as I think it looks more natural.

Thanks!

I'm keen to get the buddleia as tall as possible, is that doable whilst also trying to keep it bushy, or would I be cutting off all new growth every year.

Yes, I wondered about keeping the bed empty except for the tree. I suppose if I get a multi-stem one that would be more streamlined.

One of my buddleia is about 6ft, I inherited it in the garden, already that size. About 3ft is very old multi trunks, I suspect if you only cut the top half of last years growth, then it will get thicker and taller.
They are very tough plants, so you're unlikely to do much damage. If you don't like the result then you can prune it back again.