Update - so... I've uninstalled the WPM player/recorder from this location & removed associated folders.
Interestingly, upon re-install (run as admin) everything went well & when launching the domain account config tool, I again could see the entire dialog which allowed me to enter the account names & passwords & then validate > save these.
So far so good...
Re-running the transactions I had in place for this location I was still encountering the same issues though with proxy authentication etc, re-running the domain config tool I again saw the truncated dialog.. grrr.. not great.
I decided to try recreating the transactions themselves & strangely (IMO) this has appeared to have fixed the issues I was seeing (by that I mean they are reporting 'ok' & can access the relevant sites as needed) so now (aside from the dialog being truncated) I'm wondering whether these had corrupted somehow? As for why the dialog doesn't offer the options to amend the accounts, I have no idea (perhaps someone from Thwack can offer some ideas here?).
Fingers crossed this will have resolved the issues I was seeing for now, but I will certainly be wary of upgrading WPM in future given the work this has created for me. My suggestion to anyone facing a similar issue would be to try & recreate the transaction initially before any other steps are taken.