Apparently it is because I was in the USA during the last 28 days - there is some mosquito borne virus that can cause problems.
I had to sit around for 15 minutes before they told me that. Frustrating *grrrrrr*
The last time I gave blood was 1982! I had to stop after that 'cos I had glandular fever then after two years, when I was just able to give blood again, I caught a sexually transmitted disease and was told I should never give blood again. The rules about this particular disease have changed now and if it hadn't been for all the darned American mossies they'd have taken it this time.
I'll head off to the hotel soon and drown my disappointment in ... drinking chocolate? ... ahh well, could be worse.