Hi Jon,
Luckily you have an unusual surname for Ann;
There is this possible baptism on the IGI;
Ann SHELMERDINE
26 November 1786
Cathedral, Manchester, Lancashire
Parents, James Shelmerdine and Charlott
If this is her it would make her 25 at marriage so seems a possibily to me

Sibling ( from same batch)
James SHELMERDINE
02 November 1788, details as above
I cant find a marriage for James and Charlott +/- 10 yrs in Lancashire but they may have married elsewhere.
Do you have Charles and Ann Brown on any census records as sometimes going forward can help us with taking you back

trish