Discounted 'Advance' tickets are only ever sold as 'Singles'. There is no such thing as an Advance Return ticket. So if you're sure that you want an Open Return (rather than two Single tickets) it will make no difference whether you buy in advance or not.
Looking at the fares offered for Sunday, it looks as if a Return ticket will still be cheaper than two Singles on that route anyway. (i.e. there aren't any cheap Advance tickets). So you'll pay £20.30 if you're confident that your return journey won't break any Off-Peak restrictions or £26.80 for an Anytime Return,with it not making any difference as to whether you book in advance or not.
http://www.nationalrail.co.uk/