Tenancy agreement signed but not dated?

This could be a problem if your tenant choses not to leave and you need to take them court. With that said they have been paying you rent from the start date which you can prove. Paying rent does mean that a tenancy is created (might just not be the one they signed). Since the tenancy has long expired they are on a rolling contract.

I'm not sure if you can issue a valid section 21.
I'm not sure if a potential buyer will question it as usually they would want to issue a new tenancy themselves anyway before completion.

All of this might become irrelevant if the RRB goes through before you make a decision.

Before serving any notice have a chat with your tenants about you intentions to sell. Make sure you offer them first refusal even if you 100% know they can't / won't buy. Tell them you are looking to sell with them in suti but there might be a possibility the new buyer will want vacant possession.

Speak to your letting agent before issuing any notice. Most tenants are reasonable anyway however for your sake I have hope you have kept the rent at market rate otherwise this will cause you an issue when the tenant suddenly realises they can't afford to move.

Worst case speak to an evection specialist, good luck!

Just date the agreement 2/5/16. Your agent hasn’t been managing the property so nobody will be referring back to them when you go down the Section 21 route. And your tenant will have absolutely no idea your agent forgot to date it over 8 years ago.
(I’ve just sold a rental flat and the sales contract was intentionally unsigned at the request of my solicitor so he could insert the date in the future).

Even if you now date your copy of the TA, and could be deemed fraudulent, the one held by your tenant remains undated.

Re the comment by Dylan Morris, contracts for sale are undated, but are usually signed by the seller and buyer once the contract has been approved by the purchasers conveyancer, until there is an agreed completion date and once exchange of contracts is effected. A completely different situation with Tenancy Agreements.

First thing I would do is actually talk to the tenant.
As has been said before offer them the opportunity to buy it from you. If nothing else it's good will.
In a lot of cases you might be able to come to an amicable arrangement without even issuing a section 21. At that point I would then say just to cross the T's and dot the I's I'm going to issue a section 21.
Offer flexibility on the end of tenancy, you could go licence to occupy on a week by week basis at the end of the section which will give them far less rights if they sign that.
There is a very real possibility they don't have their original TA so then the dated thing becomes a mute point. I would also question whether signatures even need to be dated if you have both signed a TA with a date and rent has been accepted from that date.
This is a hope for the best but prepare for the worst scenario.
I would never sell a property tenant in situ if I could avoid it. You get anything up to 10% less on the sale price. So you might find the correct business decision is to offer to help the tenant financially with their move. Lose £30,000 in value or offer the tenant £5000 to go. Distasteful I know but it makes business sense.