Well, I've managed to build up quite a family behind Maria Clark now and probably going to stop there for now. Quite a sad story because by the 1861 Census, Maria and Abraham are staying with her parents and Thomas is in Brentford Workhouse. I haven't as yet spotted the 1871 records, but in 1881 Maria is a widow in the workhouse.
I managed to track Thomas' family back to Iver in Bucks., although his baptism date (1799) is slightly earlier than his age stated elsewhere. Everything else matches though so I'm pretty sure it's him.
I thought that I had their marriage in Twickenham in Nov 1823, but that Maria is Wiltshire and I was just starting to follow that up when I realised that you'd posted the 1861 record with the Stranges.
Thomas has three brothers, of whom one stayed in Iver and one moves to West Drayton (wives and families). His parents are Abraham and Ellizabeth.
So perhaps Martha leaves ASAP to escape poverty? She gets married in Cape Town aged 27, so I assume only meets James Bashforth there, although he would have been over in Woolwich doing his military training at the same time Martha is living in Hounslow.
Anyway - that's enough for now. Thanks again for your help, Rosie - I've made a lot of progress with the Bashforths now.