jaguar boiler,hot water stays on for about 30-50 seconds?

Hi Gary, it sounds like either a DHW sensor or board fault. Now I'm not suggesting you open your boiler because that would be criminal offence. Unless you are a class of person who is qualified to carry out work on a gas appliance. But if your mate who is qualified to work upon gas appliances were to open your boiler and look for a sensor with two yellow wires going to it (please make sure he has the boiler electrically isolated first) look for the second pipe from the left and follow this into the boiler he will come to a device fixed to the pipe with two yellow wires comming from it. If he removes the plug he'll see two pins. He should get a multimeter and turn this to ohms and check the resistance accross the pins he'll observe a reading if pipe is cold to the touch the resistance should be approx 16K ohms@15 Centigrade . If he gets a much higher or lower reading then the sensor is faulty. If not then you might be looking at a new board (much more expensive and not a DIY job).
Enjoy...

Mick
September 2008