Thanks for all the comments!
FWIW - Replaced both batteries with same make from same source. Terminal voltage of batteries initially not identical (one had been installed in another UPS for 24 hours). Ran unit on charge for 18 hours with no load - both batteries then reading same voltage (13.95V). Then disconnected mains and connected 100 watt globe as load - unit ran for 35 mins 41 secs before shutting down. At this stage, one battery at 11.61V and the other 11.64V - close enough for me.
Somewhat reassured by most of the comments (although I would still prefer two X 12V batteries in series rather than in parallel - but I'm not in a position to completely redesign and rebuild this UPS!).
Will not be deploying this UPS in a mission critical environment (where I have installed a very much more expensive unit with AVR and monitoring software), but there are a couple of less critical locations where it can be used.
Lesson I have learnt here is that I had better schedule a regular test and maintenance schedule for all the UPS's and keep track of all the SLA batteries.
Thanks for all the comments - most helpful!