SOLVED
It all works now! How? Short answer is:
sudo find -x /(relevant folder) -user 502 -exec chown -hv 501 {} +
(courtesy of Hal Itosis on macrumors.com)
The long answer is:
All files that were once mine (the initial admin user) with ID 501 were now owned by 502 who was a temporary (admin) user a day or so before the crash.
I went in on Single User Mode, activated the System Admin (root) account and passworded it.
I re-created my original user (chrism) as an admin and was offered my old /Users/chrism folders back. All seemed well until I noticed an increase in 'authentications' required of my actions and an inability to save in my current game. This disaster led me to check the Get Infos and found that the temporaryadmin was now these files' owner.
Initially, I, as root, dropped him to Standard user but that didn't help my status even though I was now the only admin on the block.
Before checking his UID (it would have been instructive to have done so) I, as root, deleted his user and .dmg-ed his files (just in case, although his usage was temporary).
After some reading, I ran
sudo chown root:admin /
and
sudo chmod 775 /
After this, I confirmed my status by running the command
id
which said that I was back to having UID 501 and group 20.
The worrying apps, etc, were still UID 502 and group:admin, so I ran
sudo find -x /Applications -user 502 -exec chown -hv 501 {} +
and ditto with all folders I thought remotely likely to have 502s in them. Pretty much the whole tree: /Applications/Utilities, Library, private, Volumes, individual files in the root e.g. /r* and /.*. This last found a folder tree called '/./Applications/Picasa.app/*' which is a piece of 'Applications' that was missed the first time. Run this line many times on large folders like Applications until they returns empty and then on '/' again.
I hope this helps someone, sometime.