Buddleja x Weyeriana

[MrsAntiSocial]

Does anybody know how much this grows in a season please.
I have diddly widdly ones (about 30cm high) at the moment but I have read the adult height is just under 10 feet?

That sounds about right. You can prune it heavily after flowering. I'd have said the branches grow perhaps 3-4ft in a year, but then you'd cut that back the shrub to 12 inches in March.