Almost anything! However, a tree and shrub fertiliser (Vitax Q4) is ideal

Vitax Q4 is good, no doubt.

Also extra sulphate of potash or wood ashes to get good flowers and strong stems. And don't overlook adding a little extra lime (dolomite if you can afford it!) as they do like their calcium and magnesium.

How funny, I was just going to direct H to you - together with your website which is very interesting and helpful.

No, in essence, many of them are buddleja davidii. However, in gardens we've stretched the species numbers beyond that to some almost exotic types. Railway embankments, waste ground, and other neglected sites are often rich in basic minerals which suit perfectly the requirements of many plants, buddleja included. I have a site at the garden centre which is rubble, tarmac, bits of plastic, lumps of angle iron from an old fence and pieces of concrete. The geraniums, daisies of various genera, clovers, peas (trifolium, lespedeza, lathyrus), viburnum, sambucus, crataegus, even a rose or two, all thrive. I daren't try buddleja because I know I'll struggle to contain it! Might just sneak in a small one.
Buddlejagar, what do you think of these new Buzz things? I'm still not turned on by them and have heard reports from growers that they sort of 'revert' after a year or so to something less dwarf.

Buzz Buddlejas are okay, although I have found Buzz Ivory hard to establish. They grow to about 5 foot when planted in the ground, but are kept smaller by constraining in a pot. Sadly, not so long-lived in a pot - maybe three years before the vigour is lost. I don't think they revert as they are bred from seed and not induced sports, they just grow larger than the advertising claims after a year or two.
Hardiness might well be less than the larger cultivars on exposed sites. Buzz Red (Velvet) is stunning and I think much nicer than the American Miss Ruby or Miss Molly (Red Chip).

Thank you for that. That ties in with what I've been told by suppliers. I'm happy with the Nanhos and Lochinch etc.