Well, so much for getting a Zone1-4 to Walton-on-Thames ticket.... When I rolled up at Abbey Wood on Sunday, the ticket office was closed and it turned out the ticket machines wouldn't sell me that ticket - they could only sell the ticket from Abbey Wood, which wouldn't let me travel via the underground. By the time I'd faffed around trying to buy the right ticket (including a quick look on the railforums tickets site to see if they could sell me a from-zone-4 ticket - they couldn't either), I'd just missed one Elizabeth line train, and then it turned out that the next one was cancelled. Since there was a Cannon Street service due, I figured at that point it'd now be just as quick to go via London Bridge anyway, although there was no way I could get my planned 17:37 train from Waterloo.
Amazingly, from that poor start the journey got worse ... Because little did I know that there were no trains between London Bridge and Charing Cross because of engineering works (I hadn't checked because I wasn't expecting to be on that route). Headed to London Bridge tube station to try to get on the Jubilee line, on the assumption that would be an accepted alternative route - to discover the Jubilee line gateline was closed and the Northern gateline staff who would need to let me through were all currently occupied dealing with a bunch of idiot cyclists who were trying to take their bikes on the tube!!!! After some wait for that problem to be sorted, the TfL staff were fine about letting me through.
So, I finally got to Waterloo just in time to discover that the train half an hour after the one I would have caught was also cancelled (and for some reason, although the xx:37 to Alton stops at Walton-on-Thames, the xx:07 that I could've got didn't). So I finally got to Walton an hour late. Oh and of course the trains were completely packed out with the bank holiday weekend.
Return journey a bit after 10pm - got to Waterloo to discover there were now severe delays on the Jubilee line, so I ended up with an 'interesting' route via Elephant and Castle to London Bridge.
Definitely not the kind of journey you want to repeat. But thanks everyone for all the help/advice.
Even aside from the engineering works and the bank holiday crowds making things harder than usual, you really wouldn't think it would be so hard to buy the correct ticket for what ought to be a relatively simple journey!