Yes, half the difference between what you paid and the fare that allows you to do what you want, which is the Super Off Peak Return route +ANY PERMITTED at £177.60, assuming the time restrictions suit. You may however have to try a few ticket offices to get it issued correctly; some may claim you can't go Off Peak to Super Off Peak, but in reality they are the equivalent tickets - the ECML Off Peaks are the former Business Saver, unlike most other TOCs where the Off Peak is the former Saver.
If the validity of that ticket *doesn't* suit the Off Peak is a hefty £292.80, so you may wish to have a look to see if you can beat half the difference (£64.55) with a cheap Advance.