can leaseholder buy my freehold?

Hiya,

I live in a purpose build Edwardian maisonette, and own the freehold of the building, which includes the downstairs flat. i have got new neighbours who have moved in, and they want to buy half the freehold from me.

Can they do this or do they need to wait two years before they can force me to sell? Or Do I have to sell to them now?

Any advice gratefully received!