I've taken a quick look and I think you've found the best fares already, Jo.

I could throw in some more complicated options, but the savings are likely to be miniscule and journey times may not be conducive.
One thing I thought of though. Do you not have a season from Maidenhead to London? Possibly money to be saved there for one of the fares....
As for 1st Class comps with
EMT» : Tea, coffee, orange juice, mineral water and savoury snacks. Complimentary glass of wine on evening departures. Also, an at-seat service of (paid for) items from the '
At-seat service menu'.
http://www.eastmidlandstrains.co.uk/YourJourney/pages/firstclass.aspx?page=OnboardDining