Lease extension was not registered with Land Registry

Saver83

My husband and I are currently selling our Shared Ownership flat and buying a New Build house with Help to Buy. We have to complete by August 11th because as part of the conditions of Shared Ownership you must sell at the current market value and that's when our valuation runs out. You could sell for less but we would have to take the full brunt of any shortfall in value (as opposed to sharing it with the housing association). We had the property valued February 13th, found a buyer and searched for a property that we could afford based on the equity we would receive from the sale, our savings and how much we could lend. We had our offer accepted on the new build but then proceedings were put on hold due to the lockdown. Our buyer was made furlough and when restrictions were lifted she couldn't get a mortgage. We immediately found another buyer but they found they could no longer get a mortgage either because they only had a 10% deposit, another buyer came forward but then they decided to drop their offer which we could not accept because it would not give us enough equity to purchase our new build. We were in the process of retracting our offer on the New Build because we had no buyer but then another buyer came through at the last minute who had a much larger deposit and whom were happy to move quickly and meet our deadline. Everything seemed to be going well but after the enquiries came back from the buyer's solicitors this week, it was discovered that the lease extension on our Shared Ownership flat has not been registered with the Land Registry. I am using the same solicitors I used to purchase the  flat 5 years ago. The lease was running low but was advertised as being sold with a long lease so I purchased it on the condition that it would be sold with the lease extension. I was not made aware by my Solicitors at any stage that the extension needed to be registered with the Land Registry and the person who dealt with our case no longer works at the company. The lease extension was signed and paid for by the previous owners on the day we completed. At the time, I was told by my solicitors that the extension had been carried out and there was nothing further we needed to do or worry about. As far as I was concerned, everything had been dealt with. They are now saying that the previous owner's solicitors should have registered it but there is nothing in the paperwork to suggest this was agreed by them. My solicitors found the original lease extension in their archives and have applied for the registration to be expedited by the Land Registry. This was granted yesterday but we have no idea how long this will take and if any requisitions will be raised. We are sick with worry because if we don't complete by August 11th, we will have to get the property valued again. It has already been extended once and if extended again, there is a chance that due to Covid-19 the value may have dropped and we will no longer have enough equity to purchase the New Build. We have spent thousands in costs so far, not to mention the hours and hours of time filling in applications, documents and on the phone to various parties. If there anything more we can do to resolve this? We are currently living in small, cramped conditions on the first floor with no lift (which is not ideal with having a toddler and 4 month old) so we are desperate to move and it would be devastating for both sales to fall through as a result. Our buyer will also lose out financially. To add salt to the wound, we thought we had negotiated a brilliant deal of offering to pay the full asking price on the New Build, on condition that the developers pay the stamp duty (not knowing that the government would temporarily scrap it anyway!). If anyone could advise us we would be most grateful. Many thanks.