So success!!!! There was a car in Tesla's inventory that matched my spec (M3P, White/Black interior). I contacted Fleet Solutions on the really helpful advice on here about reserving a car on the inventory, then transferring the RN over to Lease, and lo and behold, they were fine about it. They just asked if it had 0 miles on it and whether it was unregistered. I contacted Tesla, and I was able to feedback that it had 9 miles on it, and was indeed unregistered. The lovely lady at Fleet Management (They're all lovely from this point on), said that she could ask VWFS to push the order through based on the information I'd given them.
I contacted Tesla, and they confirmed that they couldn't even identify an order from VWFS that had my car in my spec, let alone a new build allocated to me, which then led to the lovely bloke at Tesla Leeds telling me that I could reserve the car on the site, then forward the RN on to Fleet/VWFS, and once the funds have been approved/forwarded on, then they'll refund the £100, and they'll be able to give me a delivery date!! They also said, worst-case scenario, in the unlikely event that they've missed somewhere that I've already been allocated a car, they'll refund the £100 anyway. My order is showing up on the site, and on my app as well!
So I now feel like the end is in sight!!!
Thanks, everyone for your help and advice