The following page on the Disused Stations website confirms that Spellow station was located on the Bootle Branch as described by the OP:
http://www.disused-stations.org.uk/s/spellow/
That web page also has this useful map showing relationships between historic lines in the area
(map credited to Alan Young)
Tunnel-lovers should take a look at Kirkdale station, where the former 4-track L&Y line, emerging from its own tunnels, passes over the old L&NWR tunnel, while the old CLC line, partly in tunnel from Walton on the Hill, runs alongside the L&Y but at a lower level. The CLC went under the Leeds and Liverpool Canal to reach Huskisson goods station, while the L&Y crossed over it at Sandhills. There was a signal box called Atlantic Dock junction where the 2 branches of the LNWR line diverged. It was overlooked by the back wall of Bank Hall depot. In the 1880s one of Sir Edward Watkin's brainwaves was for a tunnel from the CLC through a station at Central Low Level, alongside the Mersey Railway, to Huskisson, where there was a very short-lived passenger station. This would have given Liverpool a Link 90 years before it finally got it. The route from Central Low Level to Southport Lord Street via Walton on the Hill, Aintree Central and the Southport Extension, believe it or not, would have been very marginally shorter than the L&Y route, but with very inferior traffic potential beyond Aintree.
