First off, for advance purchase booking horizons, you can find the information here:
http://www.nationalrail.co.uk/times_fares/booking_horizons.htmlNow as for ticket prices I have assumed you will be working office hours in High Wycombe of 9-5 and have assumed 1h 15mins for the drive between HW and Didcot Parkway (AA recommend between 53-63 mins depending on route - I've added a bit of insurance time)
So, departure from Didcot after 1815 Tues, Weds PM and arrival into Didcot from Temple Meads for 0745 Weds, Thurs AM.
I've looked as far as possible ahead using today's booking horizon for
FGW▸ which is 24th February
DID» -
BRI» Tues 16th Feb on the 1812 - ^12.50 ADV single (^22.00 1st class)
BRI-DID Weds 17th Feb on the 0630 - ^12.50 ADV single (^22.00 1st class)
The same for Weds PM, Thurs AM. The sevices either side of the ones above show the same ADV prices of ^12.50 (^22.00) at the moment.
If you need the flexibility of a walk-up fare for your journeys the prices will be:
DID-BRI 1812 Super Off-Peak single (valid on all evening services from DID) ^16.50
BRI-DID 0630 Anytime Day Single ^22.50
Your parking at Didcot will cost you ^5.40 each day if you use
RingGo.
Hope this helps MaxP.
(EDIT: It is worth noting that the earlier you book ADV tickets for a particular date does not guarantee the cheapest fare. Yield management means that if seats remain unsold nearer the date of your particular service, the price could drop. Fares for next Tues and Weds on the same services as above are being offered today at DID-BRI ^8.35 and BRI-DID ^5.50! ADV fares really are a lottery.)