Thanks for taking the time to look those up, Alan.
I do think that Richard and Catherine's first Mary, as recorded in the Stanford in the Vale register is probably Elizabeth. Richard's will names his daughter Elizabeth first amongst his daughters, usually an indication that she is the eldest, so she would be older than Sarah, baptised in 1741 (Anne baptised February 1738 had died by the time the will was written).
There is a big enough gap between Anne and the twins Sarah and Joseph for Elizabeth to have been born, but if she was there is no reorded baptism (at least not in my OFHS transcript) and no obvious burial of a child Mary Belcher, either at Stanford in the Vale, or at Denchworth where son Joseph was buried in 1754. (I think Richard's father was originally from Denchworth.)
I don't know the origin of the information about Elizabeth Price, but an Elizabeth Belcher aged 49 was buried at Stanford 2 December 1787, so that would fit with the baptism date.