Okay - I've surfed a bit wider and found this in connection to Watermelon Buck (?!):
"That bubbling and foaming is natural. It will turn into a scum floating on top. Leave that scum on untill you are ready to drink. When you are ready to drink skim that scum off and discard "Do NOT STRAIN it" your good stuff is under it. "
It sounds like you have natural or wild yeast naturally present in that festering goo which is working hard for you, apparently wild yeast doesn't have a huge tolerance to alcohol, so I guess my recipe made something incredibly alcoholic because it added extra yeast. Some other reports suggest that Marrow/Courgette/Zucchini can be treated as pretty much the same thing for this kind of brewing.
In case you're interested in the Watermelon Buck here's the rest of what the guy said about it:
"Some people that make it in 55 gal drums

sifle from the middle. Make sure your sifleing hose is sucking from halfway between the top and bottom. This is the easy way to take off about 5 gal at the time as it leaves the top and bottom undisterbed.
Also i've only made Buck with red meated watermelons, I suppose those others will work. Over ripe melons in the field have lots of sugar in them. I think harvested melons that have become overripe from age will most likely work the same."
Apparantly you take the juice and bits from broken watermelons being harvested from the field, throw it all in a covered (not sealed) drum and leave it to work its magic in the sun.
Look forward to hearing how it all turns out!
