I spent 5 years looking for sofas for my lounge after we moved. DH reckons I'm too fussy 
I almost went with Multiyork - they seemed to have the most comfortable sofas around with the best construction but you need to wait until they have a sale on with an extra discount as you can then get them for about half price instead of paying full price.
Dsis had m&s sofas that stayed very hard and rubbery despite being told that they would soften.
My previous sofa I had made by a local upholstery company that made them - 20 years on it still looks pretty much as good as new. However the back cushions were feather and foam and the feathers are now beginning to get a bit dusty and probably in need of new pads! Unfortunately they no longer made sofas when I needed them again after moving (into a bigger place, hence need for more sofas!). So definitely investigate any local places rather than think you have to go for big names as they can make some excellent stuff and will often be able to tweak it to be just right for you.
However, I ended up with some John Lewis ones from ebay that were a year old and cost me £200 instead of £3000 - I originally bought them because we were having loads of visitors over Christmas so needed more seating and thought that I would resell them when I bought the 'real' sofas rather than have guests sitting on garden furniture again. They were in such good condition and matched my room so well that I ended up keeping them and saving my money.
So I'd say that if you do know what you want - or the brands you are interested in, it's worth having a look on ebay as there are some amazing bargains to be had. And additionally places like multiyork have some shops that have overstocks and returned orders that they sell off cheaply that if they have something you want can work in your favour. I once bought a sofa for my bedroom from furniture village - just before ds2 was born, thought it would be nice to have a sofa to sit and feed him on and escape when necessary. As I was heavily pregnant and they didn't have a leaflet for the sofa, the person in the shop measured it for me. It was really tight to my measurements for the door it had to get through but they assured me that their measurement was the absolute maximum... and of course when it turned up it was 3 cm bigger and didn't fit, even when I took some of the doorframe off. So it went back to the depot to be sold as a return (thank goodness the manager had measured it as otherwise I would have been stuck with it!).
Also there are places like Trade Secrets that sell overstocks and returns from people like M&S, Debenhams, John Lewis and lots more. Either just go and have a wander and see what they have in stock then you can take it away with you or get them to deliver it sharpish. Or they will sometimes run a list of things that people are interested in because it's in their interest to, if they know that somebody is looking for a JL grey 'Marlborough' 3 seater or whatever, they can nab it. If you do go to one of their showrooms, it's a great place to try lots more sofas than most places because all the different brands are in one place so you can compare a John Lewis sofa directly with a Debenhams one and so on. Downside is that htey don't always have everything in, but you can ring and ask if you're interested in something in particular. And best of all, prices usually start at 50% off the list price (or they did when I last went, about 5 years ago).
Good luck on your sofa hunt!