Only bad thing for you is a contract is still liable for payment with Zen and I imagine they will have covered their bases for low throughput.
I am not in contract, but if I were I'd be pursuing everything up to taking it to CISAS. In my own case I've seen a massive change in the way my connection works and I'd consider it to be worthy of getting out of contract, regardless of the lack of guarantees. An approx. 90% reduction in peak time speeds (single threaded) simply isn't normal or reasonable, especially when it worked fine for over a year prior
If it's the office service then they'd have even less of an excuse, given that there is an SLA, guaranteed time to fix, and throughput guarantees