Hi pippalew - GENUKI has a list of the districts covered by the Amounderness registration district at
http://www.ukbmd.org.uk/genuki/REG/districts/amounderness.htmlIt appears that the registers for this period are now at Preston.
Perhaps one of the map site (multimap, streetmap, googlemaps etc) would help you on the exact geography of the area.
Re the reference numbers - these are only necessary if you're applying to the GRO. You can get them from the full indices which are available on microfiche at many Records Offices & some libraries. They are also online at subscription sites such as Ancestry - some libraries have an Ancestry subscription. Otherwise you may need to ask for a look-up here (sorry I don't have a sub. so can't help).
If you apply to the Register Office that now has the registers (ie Preston for Leslie Thomas Lees) they don't use the GRO reference. They would need as much information as you can give, eg name, date (close as you can), address at the time. Some offices are more helpful than others, but the local offices don't charge for a search, just the normal fee of £7.