I would say it's called "pulling a fast one".
In my view, any attempt to enforce contract terms which are not agreed would fail. Any attempt to discipline an employee for failing to meet terms which were not agreed should also fail and would, in the event of dismissal, be termed as unfair or constructive dismissal.
Consult ACAS if the union don't get anywhere with it.
http://www.acas.org.uk/index.aspx?articleid=3971