A couple of thoughts:
1. Check your own Deeds - there may be a map showing the layout of the estate and possibly showing rear access. Check if your own solicitor made an enquiry about the rear laneway - i.e. was it in charge?
2. Go to your Local Authority Planning Office and do a Planning Search pinpointing the Detached House - you should be able to locate the planning permission and the application. This will also give you an idea as to when the house was built - i.e. more than 12 years and therefore have owners acquired ownership by adverse possession.
3. Go to Local Authority Roads Department and ask them if this laneway was closed by agreement or is it in their charge?
I'd be surprised if at the time of construction your neighbours did not kick up a fuss. It could be that the land was privately owned with no right of way granted to your terrace.
Finally, armed with this information, talk to your own solicitor and take their advices.
mf