So the section 21 is valid,
Interesting about the 21 not being valid, however I'm not sure how that'll be any use as my lease is up 20 days after said notice, so either/or I'd need to be out?
The end of the lease agreement doesn't actually mean you have to go anywhere. If you continue to reside at the property after the end of the contracted period then a Statutory Periodic Tenancy arises. You can end an SPT by giving your landlord a full month's notice, but your landlord most give you two months' notice by means of a valid Section 21. Even after that your landlord can't just turf you out. He'll need to apply to the court for a possession order, which will only be granted if he's done the Section 21 correctly and, if it eventually got that far, get a bailiff to carry out the eviction. All of that can take months. I'd say there's no point hanging on to the bitter end but you should be aware that you've got more room for manoeuvre than you might think.