OK,
Further checking revealed completely depressing news...
It looks as though, for 2 of my 3 pump/reservoir units, the seal between the glass of the reservoir and the pump top has been leaking. For one of these, the unit lost about 4cm of fluid from the reservoir; in the second it has lost about 1cm. The leaking coolant has then pooled slightly on the pump top before running down the side and dripping from the bottom on to the bottom of my pump chamber.
I've been able to clean up the mess... but it looks as though there is a deeper problem here that needs fixing. I don't think that I can wholly trust the pumps, or the little slide-on button-operated controller. I clearly can't trust the seal between pump top and reservoir.
I'm going to say that this is entirely self-inflicted. I assembled one of these units and a mate of mine attended the other two; one of the three remains in perfect working order and two have leaked... but I honestly don't know which layout the three units went in to the case.
I need to be really careful here... there's a 2000w PSU at the bottom of the case, beneath the pump chamber, so I really don't want to put this back together unless I'm 200% confident that everything is OK. My plan is to buy 2 new pumps, pump tops, and reservoirs... I'll re-use the reservoir caps. In fact, the only part of this that I'm going to need to figure out *really* carefully is how easy this lot is going to be to dismantle.