helen
I'm a bit late here, I was in hospital!
I left a post explaining this a bit back but it keeps dropping down the list and getting lost. It's all explained at
http://www.holidaytruths.co.uk/viewtopic.php?f=29&t=130309
The link in that thread gives you the dates which are open for booking for each company, you'll need Arriva Trains Wales. It's roughly 12 weeks but can be less at weekends.
The National Rail site is always the best for unbiased information but be careful to read everything, the fares it gives will be those specifically for that journey that anyone can buy (it will also do Railcard discounts off those). But lower down the page there may be a list of special offers that don't fit that pattern, such a two-for one or deals just for for certain people (ATW currently have an offer for Over 55s which is £15 return anywhere on their network but that ends before you travel even if you are old enough).
If you click
Buy Tickets on the National Rail site it gives you a list of other sites which can sell you the ticket, and most usefully it tells you which apply a booking charge and which don't (last time I looked Trainline was one of the two which
did charge extra). The tickets are the same whoever you buy off so why pay the charge? Some of the conditional offers may not work online and you'll have to call tele-sales.
And for South Wales to Manchester Airport you should ask for Wilmslow not Manchester. You could ask for the Airport itself but that might not give the Advance fares if the two companies haven't set up a link. And I seem to remember you were going to the Travelodge so would be better getting a taxi from Wilmslow