I run a shower direct off a worcester 24i, and it goes well, except every so often it doesn't heat the water until you turn on/off the hot tap several times.
Just doesn't seem to be igniting.
You can hear it making a "click Click click Click" noise when you turn the hot tap on, but it doesn't make a bassy "boom" noise that it makes when the gas ignites and warm water is produced.
This one does need some one corgi registered. Anything to do with the gas or the products of combustion should not be attempted on a d.i.y basis. It involves spark ignition and gas and thats not to be messed with...sorry
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.