The ADS has announced that the Northamptonshire NMP has been updated here:
http://ads.ahds.ac.uk/catalogue/projArch/NMP/nnmp_eh_2003/index.cfm
This looks like a promising resource although the Interactive Map view uses a fairly clunky interface via MapServer. The bit that caught my eye was the following...
"There are plans to include base mapping from a service such as Google Maps or Multimap at a later time. This will be an ADS wide upgrade to all Web GIS services to provide greater context to the spatial material."