We have used them three times; once for a bathroom (complete refit), once for a kitchen worktop plus double sink, and once to install air-conditioning.
The first stage is that someone comes round and gives you a quote, so you need to do a good spec first. Then you decide whether to accept or not. I think they now charge a fee for this even if you don't accept. You have to buy all the materials from them, and they hire local builders to do the work.
It was all pretty efficient and everyone turned up when they said they would. We got a 15% discount on all the materials for the bathroom, and the contractors were brilliant. They checked everything with us even to the height of the toilet roll holder, cleaned everything up afterwards and took all the rubbish away.
The kitchen worktop (a year later) was somewhat less successful; they sent different pair of workers, who botched the work and didn't tell us till the end, and there was a dent in the sink unit so they had to come back and replace it.
The air con installation (last year) was done extremely efficiently using a reputable local firm. I don't know if we'd have saved money by hiring them directly - probably, but it's water under the bridge now. The units were on sale price as it was the end of the summer so it seemed like good value.
The service is expensive compared to hiring tradesmen direct, but the advantage is that you have LM's customer service and guarantee if anything goes wrong. It's pot luck who you get for the work though.
Hope that helps!