I haven't been around much because the new system is so useless.
Besides, I have outstanding business to conclude in the News section.
You are right about 'implied terms' but there is no hard and fast rule that says after x weeks y happens. Every case is determinable on its merits.
However, based on the data you provided, I think you have a reasonable case - but I would go into the next meeting in an exploratory manner, rather than a legalistic one.
At the very least you should have been given some notice of the change - say 5 weeks if you have over 5 years service.
At the end of the day, the employer can change your T&Cs of employment through consultation and then by giving you notice of the change - irrespective of whether it could be 'proven' that you actually have a 30 hours contract now (i.e. implied term). So go and find out what issue the employer is trying to solve and try and explore what alternatives there may be.