Good evening, Jim ;-)
Right, I've tried checking whether it would make any sense to travel up to Norwich from Attleborough (to save you driving to Diss and paying for parking there). However it doesn't seem to help :(
If you go by Diss, you'll need to factor in about a fiver in fuel and £7.50 for a day's parking when looking at the figures below. You should also note that Greater Anglia hasn't officially confirmed its timetables on that route yet, although I'd expect that the trains shown below should run as provisionally shown on the National Rail website. (Check a few days beforehand though, just to be sure!):
To confuse things somewhat, the cheapest way of travelling on that route is to buy an Anytime Single fare out and an Advance Single back. Your options are as follows:
If you leave Diss at 0724, you change at Stratford (not Liverpool Street), arriving at 0853 and departing at 0900. That gets you to Green Park at 0926. The Anytime Single fare for that journey is £74.20 (or £48.95 using a Senior Railcard).
Leaving Diss at 0744 requires you to change at Liverpool Street, arriving at 0926 and departing at 0941. That gets you to Green Park at 0959. The Anytime Single fare is £73.60 (or £48.55 with a Senior Railcard).
Coming back, leaving Green Park at 1627 gets to Liverpool Street at 1645, to catch the 1700 train to Diss, arriving at 1822. Somewhat oddly, although there's not a Standard Advance fare available for that journey, there is a First Class Advance one on offer, at £40 (or £26.40 with a Senior Railcard).
Leaving Green Park half an hour later, at 1657, gets you to Liverpool Street at 1715, to catch the 1730 Diss service, arriving at 1902. The Advance fare (Standard class this time) is £48.00 (or £31.65 with a Senior Railcard).
So, assuming that
(a) you bought a Senior Railcard ;
(b) getting to Green Park at 0959 isn't cutting it too fine for you ; and
(c) you could guarantee to be able to leave Green Park by 1627 (because Advance Tickets are only valid on the specific National Rail trains that are stated on them), your cheapest total fare would be £48.55 + £26.40 = £74.95. (It's only 40p dearer to get the earlier outbound train). However, by the time you've added on your fuel and parking costs, you might be looking at a total around £88, which is a tenner dearer than going from Attleborough, via Cambridge.
All of which means that I've just wasted my time looking at all that lot! Oh well, c'est la vie ;-)
A Senior Railcard costs £30 for one year (either at a rail station or online) or £70 for three years (online only).
Links:
National Rail journey planner:
https://www.nationalrail.co.uk/
Senior Railcard:
https://www.senior-railcard.co.uk/