I've been slowly losing brake fluid for a while now - there's a fine film of it over most of that bit of the engine bay which probably isn't too great for my brakes on the whole, so I may as well try and fix it.

I think that its coming from the front of the two seals where the reservoir goes into the body as there was a lot of fluid there, but its hard to see for sure - i tried cleaning it all up and having someone try to push the brake pedal into the ground a bunch of times while I poked and prodded, but i couldn't see the slightest drop emerge from anywhere. That fluid had to go somewhere though, so can anyone point me at likely culprits?

The van is my daily driver, so I want to know for sure what is up and order the bits beforehand to minimise downtime.

Umm, depressing the brake pedal doesn't pressurise the brake fluid contained in the reservoir, so any leakage between the reservoir and master cylinder will be the direct result of a problem with one of the rubber seals between those components.
I've found instances where the seals hadn't been properly seated in the master cylinder before the master cylinder was pushed into place, so it might be an idea to investigate that possibility first...

....I knew that

I'll have a look tomorrow, thanks Ken

Also take a good look at the breakfluid container.. When they are older they can easily crack along the seams and leak there.... Especcialy when you rock and shake them when looking for leaks

I have exactly the same issue with mine. I have searched and searched for a leak. It must be from under the reservoir, it must be But when I run my finger around the joint, nothing.

Let us know if it was that.