Don't have a detailed answer, but we want to get one soon too, and I'll be starting with the recommendations on the "Which?" website, and you can also look up reviews of garden tools on GOY. Go to the shop bit.

Thanks a lot Weeding. I'll be checking it out.

I got mine from Homebase (their own) and thats been great

Mine is a gas powered chipper / shredder. I would say that one of the biggest things to look for is how easy it is to remove or replace the blades. Check on the availability of new blades and also see if the blades can be taken out for sharpening. Another useful feature is if the blades can be flipped so that when one side wears down you can turn them over and use the other side before having to replace them.
If your chipper is to be a gas powered one, make sure it has a brand name motor. A lot of engines these days are chinese knock-offs of Hondas or Kawasakis. Some are released under the 'Champion' name. These knock offs are junk and you can not get replacement parts. The first thing to go is usually the rope rewind starter. You are better off spending the extra money and getting a quality motor. Believe me, I'm a small engine mechanic and I know what I'm talking about.
If you are thinking of an electric one then I'm afraid I can't advise you about the motors on those.
Mine is a Bear Cat brand which is affiliated with Echo products. I don't know if these are available in the UK but I would highly recommend it if they are.

Ah Gilli, took me a minute to realise you meant 'petrol,' 'gas' in the UK is comes from the north sea and is used to fire central heating boilers and cookers. I'd agree that for any serious use you need a petrol driven shredder. Mr MB was given a shredder when he retired and though it is ok a bigger petrol driven one would have been far better. One thing to check is that the feed tube to the collection bin isn't going to get jammed the design on the one we have means you can't actually fill the bin because the shredded (well - sort of) matter does not drop down cleanly.

Just to say, for semi-professional use, you need a REALLY good one. Definitely see it being used, with large and difficult stuff. Over the years I wasted lots of money buying a small underpowered petrol chipper, which was always clogging up, deafened me with its racket, and absolutely drank fuel. I went back to electric shredders which were much nicer to use, but took too long to deal with any sizeable amount of shreddings.
I notice the landscape guys here use a massive machine which reduces whole branches to chips in a few seconds. But of course, they cost a fortune.
I've given up shredding for my own amateur use. I just pile up the twigs and small branches and leave them for a year. Or drive a very old rotary lawnmower over them. Once the twigs have rotted a bit, they make a good mulch. Anything larger we cut into pieces to burn on the wood stove.

LOL MG....I'm "bilingual" but sometimes I forget which "language" to use. My apologies....yes, petrol powered....not natural gas powered....although I bet they would work well!

Thank you all for your replies. All very helpful

Cheers