Sometimes, you may be unable to attend a disciplinary/grievance/etc meeting at the time/date scheduled by management. Because of this, an alternative date/time can be arranged. It seems to be standard practice on London Underground that management set the alternative, which could cause you problems if neither date is suitable.

It should actually be you - the member of staff - who suggests the alternative date/time. See the ACAS guide to discipline and grievances, p.19, here.

I have asked the relevant RMT representatives to raise this issue at the working group with management that deals with such issues. In the meantime, if management offer you two dates, neither of which you can make, suggest your own alternative and show them the ACAS reference if they doubt your right to do so!