Hi BP as no one has answered you - this is what I do to my box its what the RHS recommend.
Water well till established, every year I apply a general-purpose fertiliser - 2oz per plant in early spring and a add good mulch (the RHS say 70g per sq m (2oz per sq yd)
I think if you feed now you might get some lush new growth that suffers over winter?

Hi Drc,What a brilliant answer, I will take your advice and leave them for this year. Thanks very much. P.S who keeps messing with the letters on the keyboard, it takes so long to find them?

lol

On the other hand I've never touched mine except with clippers and they are growing quite happily. Drc is right though about too much lush growth at this time of year.

HI Cutsandgraz,Thanks much appreciated, barmyphil.