Planting out is when the soil is warm enough to sit on bare-a....whatevered!
Potting on depends on the root ball in the pot. If it is so 'tight' that you cannot get water in it, then pot it up but not to much bigger a pot, it will not grow new roots for a while yet. All you are doing is creating a space for water to go into to seep into the old compost.

O.K Thanks

I'd get it in the ground asap assuming the soil isn't frozen and otherwise keep it outside in a sheltered postion.

A number of my customers have made the mistake of buying out door plants in cold wearther,kept them indoors until the weather improves a bit and they then die of cold shock on planting outside.

Ah, yes, well perhaps I should have said keep it somewhere cool rather than in the house or even a warm glass house.

It has been planted out.

Good luck with it then! They're tough, I'm sure it will be fine.

Thanks Beattie

I bought some Buddleias last last year, large plug plants. They're in the conservatory just now until the final frost have past (which up here could be April) then I'll pop them into planters until they're big enough to go in the garden.