At the Expo today (Friday), a team from UCL was beginning to recruit volunteers for a study looking at why people collapse in marathons, by using Heart Rate Variability.
Participants need to have an ECG and blood pressure taken (no needles!) both at the Expo and at the finish line - you get to have a lie-down in the Elites tent while someone fetches your kit bag for you.
It seems like a good study, and though I've nothing to do with it (except as a participant now), I thought I'd plug it here as I know they were struggling for numbers.
You may have to hang around a bit at the expo, so if you're interested, make yourself known as you come in (they're on the left, just after you collect your timing chip, amongst other clip-board-bearing people) and maybe be prepared to come back after a look around the Expo.