press the function button four times and you should get a tap symbol on the top line. this will heat the water for 1 time only, then revert to timings.
Workhard
if it is a Worcester you want, I suggest you find a Worcester Acredited Installer (WAI), they have been through specific training and will provide a Full 5 year Bosch warranty. (6 if you have it fitted before 31 Aug 2011).
good luck.