so... it turns out that the water was leaking in on the passenger side of the boot. The mechanic took the car apart to check the drains and they were working okay. But, he did find that there are little gutters that run around the the edge of the soft top where it joins the bodywork and they were full up with 'gunk' - much like house guttering would be. When he poured water down over them they were overflowing and draining straight into the boot. When all the 'gunk' was cleared out they drained away perfectly through the drains.
hope this helps anyone else who might be having the same problem!
