If you'll definitely be getting that train then I'd hold fire for a couple of weeks before booking, as it doesn't look like advance ticket have been put on sale yet for 28 Aug. According to the graphic on the front page of
FGW▸ 's website (which is admittedly a little out of date sometimes) tickets are currently available up to 11 Aug only.
Keep an eye on this page for updates on advance ticket availability:
http://www.nationalrail.co.uk/times_fares/booking_horizons.htmlEdited to add:
And if you wait until Advance fares are released, you may be able to pick up a first-class advance ticket which may be a better deal than standard advance plus weekend first. Remember that there's no rush for you to buy the super off-peak ticket: if you wanted to you could buy that at Paddington on the morning of travel seeing as it's a walk-up fare.