want to sell freehold - do I have to give notice of first refusal?

Bought a flat in 2006 and acquired the freehold of the building with the purchase. The building is split into 2 flats. We have been letting out our flat since 2008 and have just put it on the market. I made an informal offer to the upstairs leaseholder for a share of the freehold a few weeks ago and she is not in a position to do so at the moment. My offer was at a reduced rate, considering her lease is now at 74 years.

So we have extended the lease on our flat to 125 years and have spoken to a few investors who are interested in buying the full freehold at a price which is significantly more than the offer I made to the other leaseholder. Obviously this is an attractive prospect to me. I have no interest in maintaining the freehold once I have sold the flat.

I have consulted 2 solicitors about whether I need to give an official notice of first refusal to the other leaseholder before selling the freehold to a 3rd party and I am getting conflicting advice about whether it is necessary considering we have 50% interest in the property. I want this freehold situation sorted before I sell my flat so would have to wait for another 2 months before we could exchange contracts.

So I please ask for your opinions on whether you think I have to serve notice and wait or whether I am safe to sell to a 3rd party now.