Boilermate 2 - Assistance required

I am at my wits end with our Gledhill Boilermate 2 and would really appreciate some assistance:

For whatever reason the thermal store fails to fire the directly connected Potterton Suprima 60L when the system is drained of hot water. For example, if the radiators come on early in the morning, during a very cold night, to heat the house - the boiler will not fire straight away and there will be no hot water at all for a bath or shower, until the boiler has fired again at a seemingly random period in the day.

The boiler will fire and produce hot water for the heating and bath, but it takes several hours to register that there is no hot water in the system and never in enough quanitiy to do everything at once. It's driving me mad, I have done everything I can think of to find the source of the problem; I've checked the top tank for water (it has a float and top-up mechanism fitted, so it's never empty), the boiler has been regularly serviced, the house thermostat has been regularly checked and new batteries fitted every month (regardless of age), several plumbers have been around over the last six months and none of them has been able to accurately identify the problem and the last one suggested that we rip out the old one and replace it - I am obviously very reluctant to do so, due to the cost and the fact that no-one has ever convinced me that they know what they are talking about (I am a construction professional of twenty years experience).

Any assistance that you might be able to provide would be greatly appreciated.

Regards

Ben

Go through the posts on BM II's
More so ref the top tank.
Checking if unit takes water.
Also has the room stat been changed?
Check with neighbours to see what they have fitted.
If changed some one may have altered wiring etc.
Put the dial on front of the unit to full winter setting.
Need to check if boiler is being told / asked to fire.
Also check boiler pump.
Mike