Thanks everyone for all your comments.

In response to Billy's question, the root ball was almost bursting
from the pot - in fact, I think it had actually split the plastic
container in one of the plants. And thanks for the Schultz Starter
Plus Root Stimulator recommendation. I'm based in the UK though and a
quick Google search doesn't bring up any UK suppliers although I will
try to find a similar alternative.

JimR - I don't think the soil is that bad but we did add some improver
to it when we planted them but we haven't mulched which I guess
wouldn't hurt. However, there used to be a pretty established (aprox
15 yr old) ash tree in the same spot until we had it felled and the
stump/roots ground down. I reckon that probably could have something
to do with any nutrient deficiency in the soil.

I also don't think they have been under/over watered. We did water
them when they were fist planted but have left them be since then
(about 2 months) although during that time there's been hard frosts,
mild sun, snow, terrible downpours and dry fortnights so we've had the
whole range of weather!

I guess aside from wanting to protect my (not inconsiderable)
financial investment, I am glad to hear that this probably doesn't
mean these lovely plants are dying!

Thanks again for all your comments - they're much appreciated.

Jo