There is a Martha Thomas, daughter of George and Martha, baptised 29 Jul 1765 at St Mary Redcliffe, Bristol, which is a few years earlier than my Martha is supposed to have been born. I just mention this because Findmypast says there is a marriage between a James Blacker and a Martha Thomas 14 May 1775 at Long Ashton, by banns, with the banns being read 13 Apr 1775. So possibly James Blacker was my Martha's stepfather and she took his name? Trouble is I can't find an image of this marriage record to see whether the bride was a widow or not. Ancestry has the Long Ashton marriages for 1775 but they go straight from April to July and there is no page missing. The banns are dated 30 Apr, 7 May and 14 May, so could it be that the final date of the banns has been indexed as the marriage date and they didn't actually get married? Unfortunately the banns record doesn't show whether Martha was a widow.
https://www.ancestry.co.uk/imageview...cklabel=Return
https://www.ancestry.co.uk/imageview...cklabel=Return
There is also a Martha Blacker, daughter of Abraham and Mary, baptised at Clutton, Somerset, 22 Aug 1762, even earlier than my Martha was supposedly born. There is a James Lane / Martha Blacker marriage there in 1785 and the Lanes were still having children baptised after my Martha Blacker married Richard Oswald, so I think we can rule that one out.
Ooh, I just thought - I can order my Martha's death cert cheap from the GRO now. I don't suppose it will help me trace her back but it will give me the date and cause of death at least.