Hi All
This very complicated family is starting to make a bit more sense to me now. It explains why Martha referred to her father as a Watson, but other people on ancestry have her as a Margesson (and Ann Elizabeth used it as her childs middle name). So the family were either used the double barrelled Margesson Watson or sometimes just used one surname!
Martha's son, Charles Thomas Franks (born 1871) was in the Royal Marine Light Infantry, so the military connection is there, from grandson to son to father.
George & Esther seem more likely contenders for being Martha's parents, than Robert & Caroline - I'll have a look at the census.
Also, on the other side, Charles Franks (born 1839 Wolverhampton) had a sister called Elizabeth, so the Jemima Magness might be their grandmother -mother of Charles Franks born 1810 (Preston). Lots to investigate - thanks!