I was wondering about that! How long does it take that quantity of water to collect?
Stony, I think you are the only Mk II owner around here these days. That's good, except there's no one to give you firsthand, model-specific advice. But we'll do the best we can.
The first thing you need to do is find that leak!! I kinda doubt it's the window, but hopefully I'm wrong, as that would be a relatively easy fix. Can you have someone hold a garden hose to the outside of the window and see if you get water inside? Likely or not, the window is where I'd start hunting.
After that I'd be looking directly upward, toward the poptop hatch. Sadly, your Mk II hatch is entirely different from any other C22, and few if any of us have ever seen one in person, including me. So pics there would help.
I would also throw a tarp over the boom until the leak has been found and fixed, in the hope of reducing further damage.
Also - just spitballing here - it's possible that owners of C25's or 27's in Mk II configuration might be of more help. Just a thought.