According to Google the postcode is L20 5DG
And the result puts an arrow on the A566 bridge over the railway midway between Seaforth Stn and Bootle New Strand.
There may be more info available in John Gahan's book Seaport to Seaside.
Which I will dig out in a minute.
--- old post above --- --- new post below ---
On p28 2nd para
gives a description of a journey along the line.
At the junction where the Southport line diverges from the Wigan and Preston lines, around the curve, passing, on the right a small engine shed, and shortly after passing under Bank Hall Lane bridge, draws up at the small station of MILLERS BRIDGE
To me this is the location of the current Bank Hall station.
The info I posted earlier re the postcode appears to be wrong but not too far away.
The name Millers Bridge is more readily associated with a bridge over the Leeds Liverpool Canal nearby