The ads you see for cheap ticket sites are misleading. They don't sell any tickets which aren't available through the National Rail website. (The only exception might be on Megatrain services, but they don't operate on your route anyway).
Further, some of the cheap fare sites add a booking fee to their prices, making it more expensive to book than going through the National Rail website. To make matters worse, it's not uncommon for the independent sites to make errors, resulting in passengers being issued tickets for non-existent services (or similar errors). I used to run a railway station and I've seen plenty of 'duff' tickets issued The Trainline and similar sites.
The full ('Anytime') return fare from Chester to London is £227.00. If you travel off-peak, the 'walk up' ('Off Peak') fare (which you can buy at the station, as well as in advance) is £70. Discounted 'Advance' tickets aren't available on all routes (or their availability is extremely limited). I've been unable to find any such fares on your route but you can check your specific travel requirements here:
http://www.nationalrail.co.uk/
However you should also look into 'ticket combining'. While I can't find any cheap fares from Chester to London, there seem to be plenty from Crewe. So you might find it cheapest to book the two sectors separately. (Note that there are no return 'Advance' fares. You have to book a single ticket for each direction of travel).
Chris