Leasehold

Hi Alexandra - that could be the issue and I assume you own both the freehold and leasehold for the property and there's another lease re the solar panels.
If you are selling the property then the buyers would need both the freehold and leasehold transferred and if the latter is still in your original seller's names then that will be a major issue
One option is to ask the seller's to now transfer the leasehold to you but clearly 8 years on that may not be an option. Another maybe to try and determine the leasehold, close it, based on what happened in 2015 and since.
But very much one to pursue with your conveyancer as to what you can do to resolve matters. We can't advise you on that as you/your conveyancer will need to look at what has happened and what the legal position now is

Adam, a huge, thank you for coming back to me so soon, I appreciate this so much.

how would I go about determining the leasehold? Just for your opinion, I am wondering which is the quickest, easiest route. We should have completed the house sale on the 1st of November. We are packed up and ready to leave. I also have three children who are waiting to start a new school.

I believe this situation came to light in July and has been ignored. Since obviously at the time I had left this in the hands of my solicitor.

Alexandra - which is the quickest and easiest will depend very much on the facts and what legal advice your conveyancer offers. With regards the 'transfer' I mentioned that would be all about tracing the seller and getting them involved.
If that's not an option then our PG 26 explains more re 'determination' an I suspect, but very much for your conveyancer to advise, that section 5 may apply. You would then have to demonstrate how the seller/previous owner had in effect 'surrendered' the title/tenure to you 
Neither route/option will be lightning quick I suspect so the urgency should focus on discussing options with your conveyancer and to get things moving depending on that advice. 
I'm really sorry that I can't offer much more than that but it seems the conveyancing ball was dropped in 2015 and naturally not picked up again until the buyer's realisation that it was still bouncing now. Hopefully your conveyancer can move swiftly re any advice they can offer now