This isn't necessarily the cheapest possible but:
Rishton to Preston Day Return £7.30
Preston to Crewe Off-Peak Return £24.50
West Midlands Day Ranger £21.60 -
see map for validity
Total £53.40 (The Rishton - Northampton Off Peak Return is £66.80).
You must travel via Crewe. You could use the following trains with the above tickets:
08:24 Rishton - Preston (Lancs)
09:17 Preston (Lancs) - Birmingham New Street
11:14 Birmingham New Street - Northampton arrives 12:20
17:55 Northampton - Birmingham New Street
19:20 Birmingham New Street - Preston
21:22 Preston - Rishton arrives 21:53
Are you going for the Accrington Stanley match by any chance?