The first thing you need to do is decide on a shortlist of sets you like - ideally having seen them in use, so you're happy with the picture / sound quality and the aesthetics of the set.
Then I'd suggest you make use of a price comparison site e.g. pricerunner.co.uk (there are others).
This way you can get a quick view of who's doing the best deals.
Don't forget to look at the total price i.e. including delivery, not just the 'item price' - delivery can be quite expensive and may offset a large part of your saving.
Also consider John Lewis include a 5 year guarantee in their prices - is that important to you ?
Two final pieces of advice
- if you're unhappy about dealing with a company you don't know very well then see what feedback there is about them. But if still unsure then don't use.
- If a price seems (particularly from a little known source) to be 'too good to be true' then it probably is.
Hope this helps.