The endoscopy biopsy discovered GVHD (refer to Wikipedia for the deets). So now they now what is causing all the trouble.
Prednisone (a steroid) will fix all GVHD problems apparently. So my diarrhoea, skin rash, temperatures, lethargy, red eyes etc should see some return to some shade of normality over the next 5 days.
What a relief. It was very hard work maintaining some form of sanity through the last 4 weeks.