Yes, Skyline, this is what I was trying to discover.
I think it depends the way your contract is written. If you are paid hourly, yet it says you work 11 sessions per term each session of x hours, but it doesn't specify when the sessions will occur in the term, then they would probably be able to call the shots on when the sessions are. So if they have removed one, they owe you one session (not dribs and drabs of extra hour here and there at their beck-and-call, to make up the missed session).
I think that I am really saying, it depends on how far you want to push this - it was definitely wrong that they tried to deny you pay - because that is definitely unlawful deduction of wages - you were ready and willing to come in but they closed your workplace. The employer now seems to have realised this (or hasn't realised the legal implication but just thinks it would be unfair), so you aren't losing pay anymore. If you travel a fair distance to work, it may suit you to tag the odd hour onto an existing session to make up the hours - rather than demand one whole extra session.
However if your contract clearly states you will be paid for x sessions per week, or 24 hours per week or whatever, that appears to tie down the employer on that lost day, you could keep soldiering on. I understand you comment about goodwill - but sometimes it is better to consider the broader picture of your role / how you are valued / whether it is better to chalk this one up as a future negotiation point.