I can't see that this is actually what happened.
Either this is a reverse, or parts of the story are missing. An estate agent, buyer and sellers soliciors, and a buyer would not all sit quietly for months without mentioning that notice needed to be given to the tenants. Everyone knows you cannot exchange with tenants still in the property and that Covid means the notice periods are longer and so this stuff is even more important now than usual. Already, it was an unusual situation that most estate agents and solicitors would advise against - a property being marketed and a sale agreed whilst tenanted. At the very least, notice would be given immediately, especially given they had only just moved in and would be unlikely to want to go early.
Op, what have the solicitors and estate agents said when you have approached them about the fact formal notice hadn't been given?
The standard paperwork for mortgages and to create a memorandum of also and to do the initial solicitor start-up all involves disclosing if there are tenants, and if notice has been given. It couldn't just be passed by without anyone expressing concern that there were tenants still there and no notice had been served. It just couldn't.
Did the Landlord actually market the property Op and did you see it via an estate agent or was it a property you loved and you simply approached him for a sale? You haven't mentioned estate agents.
You certainly won't be getting the stamp duty holiday and unless you can get an enpxtension on the mortgage offer, that money is gone. It isn't down to the ten ants who had just m odd in and not been served notice. You now say they have been served notice 2 weeks ago and are saying they can't find anywhere....well it's not bloody surprising is it, in the space of 2 weeks and they were under no obligation to even consider lookimg elsewhere before notice was given and before they got close to the end of the notice period. I cannot see any gripe against the tenants at all.
You seem determined to hold on. Well you can. You might have to pay another mortgage fee (or you might get an extension but as the property market is likely to decline after the end of stamp duty holiday, they might want another valuation done or not be prepared to extend becaue of all the uncertainty) and you won't get the stamp duty holiday and you might find you can't exchange in 6 months becaue eviction proceedings are needed.....or that actually this seller isn't really interested in selling at all but is just pissing about because he needs to appease a mortgage lender he is in arrears to and who is threatening repossession if he doesn't sell first.....but doesn't really want to.
You can hold in and keep spending more money and have an indefinite end if any end in sight. Will you stick with your solicitor given their lack of advice is what has got you in the mess, according to your story.