I would imagine if it looks healthy it probably is . your not going to get results strate away as it will take a year to settle in . it should do a lot better this summer . is there privet growing near you anywear ?

A year isn't very long, especially with the winter we've just had, and I assume you bought the bulk hedging plants, which often aren't very big to start with. Clay soil won't bother it, but you're not going to like what I'm going to say next, lol! As you want a good, thick, healthy hedge, you need to go outside sometime in the next 2 weeks and cut back all shoots by half to two thirds. I know this seems counter intuitive because you want quick height, but the plants will respond by producing new, vigorous growth with twice as many shoots. Apply Growmore or similar (Vitax Q4) around the base and rake that in, or turn into the soil lightly with a garden fork. This pruning back by half should be repeated next year, and so on, until the hedge is at the height you desire, when you then just keep it clipped in the usual way.
The other factor is proper preparation - if you did not dig over the area thoroughly, incorporating composted materials as you went, this would also account for slow growth.

A complete answer there Bamboo. All I would add is that the roots will take about two or three years to grow big enough to support its regular growth rate so just give it a chance.

Lovely avatar, Fractal - where you been all winter, good to see you back.

Thanks Bamboo. The Hepatica is in full flower at the moment in my garden so thought it would make a nice avatar.

I've had a computer problem for nearly six months! Hence the few appearances on here. Left talk talk, now with plusnet.....say no more!

Cheers.

just what i was thinking bamboo lol x .

Thanks everyone for eveyones input, I really appreciate it!!
Bamboo - you are right about the bulk hedging plants, they were quite small. when initially planting in early march last year the clay was very thick and difficult to excavate. I did add compost to the hole before planting. Maybe the hole could have been deeper or more compost? I had some plants left over which I put in pots just in case some of the main ones didn't grow. The ones in the pots have grown far quicker, hence one of my concerns regarding the clay. Do you think it would be worth digging them all up, make bigger and deeper holes with more compost in to give the roots a better chance in the short term so when they do tackle the clay they might be stronger and a bit more mature. Thanks again!

No, I don't, specially not at this time of year - what you could do is add a mulch of soil conditioning compost or any other composted material you can get hold of, such as well composted horse manure from a reliable supplier (there's been a bit of a problem with horse manure to do with a certain herbicide). I don't mean potting compost, as in multi purpose or any others, but if you add a good 2 inch minimum layer of composted material beneath the plants (do the whole row, not just round each plant), with Growmore or Vitax underneath (applied first), that will eventually be taken down into the ground. The slow growth may also be partially to do with water supply - we had some pretty dry weather last year, and it may be that you watered the ones in pots whenever they needed it, but the ones in the ground didn't get so much.

bamboo is right but time is the real key .