The Super Off Peak Return is £47.80.
Super Off-Peak tickets issued
for journeys commencing WITHIN
the network area to
destinations outside the
Network Area with travel via
London are valid by any train
to London to connect with
trains from London.
So this ticket is unrestricted between Cambridge and King's Cross, and then you can get any train from Paddington between 0935 and 1550, or after 1920.
The return journey can be made any time within 1 month and the only restrictions are in the morning peak on the leg of the journey between Pewsey and Paddington. There is no restriction from King's Cross to Cambridge.
If you leave Cambridge at, say, 0745, and leave King's Cross at, say, 1735, it would cost £32.50 for an Anytime Day return, or £37.00 for an Anytime return, and that only gets you to King's Cross, you then would have to get to Paddington and then on to Pewsey,. Therefore splitting is not economical for such a journey.
If you wanted to split, if you travel off peak and return the same day you could get the price down to £15.80 (£14 on weekends) to Liverpool St, then use PAYG to Paddington, then pay £23.00 for a Paddington to Bedwyn, then £4.50 Bedwyn to Pewsey. This saves about £4 and would take ages!
The time restrictions only apply on weekdays.