Now that the images of the PR entries for Formby are available, I have found that the baptisms of Thomas and Alice's children have extra info such as "de Mile" or Mill, and then there is a Thomas, son of Thomas Rymer, Millhouse, baptised 2 Jul 1746, and a John, son of Thos Rymer, mill, Ainsdale, baptised 5 Nov 1752. So my Thomas was a miller, and his baptism isn't the one which Merry found, but probably 20 Jun 1797 - that one says Ainsdale but doesn't say whether he was a miller though.
I also found a PDF of research which someone has done on Thomas Rimmer / Rymer, miller of Ainsdale, which is going to help a lot but is going to take some time to work through!
http://tsgf.pbworks.com/f/Rimmer+of+Ainsdale.pdf