S.21 notice received- can tenant leave early?

[Likelystory]

I think I already know the answer to this (on the basis of commonsense) but thought I'd ask anyway - in case commonsense and the law are at odds (won't be the first time ).
I am in the process of selling one of my properties and am expectng to complete by the 29th April.
My buyers are a lovely young couple that are currently renting and they have a 12 months ast which was taken out last june which was due to expire on the 20th June 2005.
After being asked what their plans were (and saying that they would be expecting to move on at the end of the tenancy) they were given notice to quit (section 21 I believe) around Christmas time with a expiry date which takes them to the end of their tenancy in June.
Obviously if they could leave earlier without penalty it would help them financially (as otherwise they will be paying rent for their old property as well as the mortgage for their purchase from me).
As they are still within the original term can I assume that, regardless of the fact they have been served with a notice to quit, they are tied to the original term?
I believe that when they enquired they were told by the Landlord that the property is to be sold but the buyers are unable to move before July so there is no chance of them being able to leave sooner.
Sorry this so sketchy, I am just trying to help them if possible and am a little worried that the landlord is spinning them a yarn about not being able to mitigate any losses (this is a very popular student area of Newcastle Upon Tyne and I would expect the last thing a landlord needs is a void at this time of year so I can understand their viewpoint completely).
Any viewpoints would be gratefully received

[MrWoof]

Normally, after the first six months, a tenant only has to give one month's notice, its the landlord who must give two months. There is one month's rent at stake here, purchases can and do get delayed so it might be insurance if they stay on or agree with the LL that they can stay if the completion is delayed. (providing he hasn't re-let the property).

[Paul_f]

I hope Mr Woof doesn't advise many landlords or tenants on this subject. What does he mean by "insurance"; it's nothing to do with it , nor has giving notice after 6 months during a fixed term of 12 months!!!!!!.

The poster put the question very well and gave excellent details, but the scenario together with options are as follows:-

1. The AST is for 12 months therefore is legally binding on both parties should either side wish to enforce it.
2. The landlord has issed a S.21 Notice already which is fine, providing its the correct version 1 (b).
3. If the tenants wish to move early then they can without penalty only by mutual agreement with the landlord, no matter what the AST states.
4. If the tenants tell their agent/landlord they want to move early then they must be allowed to do so, but....they will be responsible for the rent until new tenants are found, and the landlord's costs. It might prove to be just as expensive as paying the rent for another two months.
5. If the tenants want to complete their purchase by the end of this month there is nothing to prevent their doing so in principle, but I completely understand their predicament.

[Likelystory]

Many thanks for the responses, much appreciated .
Mr Woof, I recognise your point about having a bit of extra time/leaway with the rented property but so far all the searches & survey has been done, mortgage offer is sorted and we have exchanged contracts today with completion scheduled for 29th April so, fingers crossed, all will be well.
Having had a rummage around on the Land Registry website it looks like my buyer's rented property has very recently changed hands.
They have an letting agent who has previously told them it is not possible to leave sooner than the end of their tenancy - but there is always the possibility the new owners would be amenable to having possession sooner, so I will suggest it might be worth asking the question directly.
Thanks, Paul!!
Obviously, I don't know whether the buyers have purchased in order to occupy themselves or if they have bought in order to continue letting it out.
No doubt if they are buying to let then they would be happy for the tenancy to take it's course but, as the saying goes in my neck of the woods "Shy bairns get nowt" so if it is possible to find a way to contact the new owners I would say it would be worth finding out how the land lies.
So I will suggest that my buyers try to contact their 'new' landlord and just ask - the worst that can happen is that nothing will change and they have to continue to pay rent until the end of the tenancy (and pay the mortgage for the lovely flat they are buying from me).
But they might get away with being able to leave sooner, if the landlord is happy with this, and the few hundred pound they save will be handy for them to spend on making their new home 'their' home.