marymoocow great detective work by your husband
Hopefully your sellers can sort something out soon for you!
I have a suspition that we are about to head into the same waters.... we are pretty much ready to exchange except for waiting on two outstanding enquiries. The first is an indemnity insurance because the house has been extended over the garage and the covenants say no changing the look of the front of the house. The extension is over 10 years old so solicitors think low risk and that an indemnity insurance will sort it.
The second is that we are still waiting on building reg certificates for two extensions (one over the garage and one at the back of the house). I have checked the local government website and planning permission was granted. Our solicitor is chasing our sellers solicitors for the building reg certificates but they have gone too quite for my liking... so my guess is that they dont have them 
Our seller has only been in the house for just over two years and the extensions were all done before she moved in - I would have thought her solicitor would have checked this before she bought the house?!?!
If they dont have them we have to decide what to do next... if we ask them to get retrospective certificates they wont pass as the work was done over 10 years ago so wont meet 2014 standards. We could look at another indemnity insurance but that doesnt guarantee that the work has been done properly...
In the meantime our bottom of the chain are pushing for exchange this week and completion next week.
Why do solicitors take so long!!!!