My understanding is that the only way you can guarantee to overcome the potential problems of a lease getting near to the end of its life is to use 'staircasing' to eventually buy out the HA's share of the property. Otherwise any other arrangement will require the voluntary agreement of the HA, so you should start by simply asking them if they'll actually consider extending the lease (as they're not obliged to extend it). Such a request for information shouldn't cost you anything.
If the HA is prepared to consider extending the lease, you can ask them for further information about the potential costs and the process involved. (Again, there should be no need for you to pay anything just to obtain such basic information).
You'll then be in a better position to see if it's worth pursuing the matter any further.