James,
The block may have bought its freehold but don't assume that this particular owner was part of the deal. If they WERE, then they will be a shareholder in the freehold company and will easily be able to extend the lease. LIkewise, if you buy the property, you can also extend the lease.
There is a cost, only a few hundred pounds, and something that you can easily negotiate the seller to pay.
If this owner is not a shareholder, you should insist they serve a Section42 notice and extend the lease prior to your committing to a purchase. The law says you need to have owned the property for 2 years before you can do this, but the current owner can begin this and transfer the process to you via a deed of assignment.
We at Leasehold Solutions (www.leaseholdsolutions.com) frequently find that 72 years is short and many lenders won't be keen on lending money on such a short lease. So be wary.
If you need a solicitor or surveyor, there are vetted specialists at www.alep.org.uk, the Association of Leasehold Enfranchisement Practitioners.