The principal of Adverse Possession operates here, but there is a difference between the situation depending whether this land is registered (at LR) or not.
If the land was registered then, since 2002, the process has been that a claimant of the land can make an application to LR to claim the land after 10 years possession, LR write to the owner, if the owner then does nothing for another 2 years the claimant can claim the land as his own. The bit I'm not sure about is what happens if LR can't get in touch with the owner at this point.
If the land is unregistered or the claim occurred before 2002, the process is similar except that 12 years minimum possession must occur and there is no step of needing to write/find the original owner.
You can check the current ownership of the land, if registered, by going here, typing in the postcode for one of the bungalows, paying �6 to download both the Title and the Title Plan. The Plan will show you (in an adequate level of detail for what you want) whether elements of your father's former land has become subsumed into this bungalow's plot. If it has happened for one it has probably happened for all.
I guess you will either find that it is registered as part of the new plot, in which case not much you can do (unless you can prove it was claimed after less than 12 yrs - unlikely). Or you will find they are using it as garden but haven't actually claimed it. In which case, find the deeds (if unregistered) or get the details (if registered) of your father's plot.
http://www.landreg.gov.uk/