The entire built-up area of Wendover is separated from the HS2▸ (The next High Speed line(s)) route, a chunk of which will be hidden in the Wendover Green Tunnel, by the A413. Planning application 25/00406/APP shows that the chamber was proposed to be located on farmland to the NW of Wendover and it appears that it was the Kings Farm track which could have been extended to provide access.
Personally, given the cost so far, I would have approved a new direct access from the A413 with a bridge over the railway.