Steve, Mary Crutch born c 1787 had 2 illegitimate children Rosetta and Mary jnr, and never married, that I know of.
Mary Crutch jnr was born in Chearsley and had an illegitimate son, George Crutch/Crouch before she married James Piddington. In giving her father's name as Jonah Rose it may have been correct, or complete fabrication. As you say, unusual perhaps to completely make up a name.
But looking now at Great Milton parish registers there was quite a large Cruch/Crutch family as well as the Martin already mentioned in my original post. No Mary Crutch c. 1786, unfortunately.
However, interestingly there was another illegitimate child, Priscilla Crutch daughter of Mary Crutch, baptised 15 Feb 1807, Great Milton. 'My' Mary would have been c. 20 then, so Priscilla, who is easily traceable in census, may have been half or even full sibling to Rosetta and Mary.
The more you look, the more you find, solve one problem, uncover two more, etc.
Regards Margaret