I too have a Worcester 9.24. It works great on C/H, but when on Hot Water, it will run for a while if the tap is set to a trickle - trouble is that when the tap heats up, the trickle reduces and boiler stops delivering!
However, having run the boiler with the outer and inner casing removed, I have discovered that with full demand Hot Water, the burner is getting too much gas, which means flames flood the combustion chamber and lick out underneath, thus depriving the pilot light of air, which puts out the pilot light and eventually shuts the boiler down.
So far, I haven't found anyone who knows how to adjust the main burner gas delivery rate in H/W mode.
If anyone knows how, please e-mail me: bobandnen@ directsave.net, and I can then give my pet corgi installer (who only wants to fob me off with a new boiler!) detailed instructions!!
Thanks.
Bob V, September 2008