how can i fix my low pressure?

Hi

I have a Worcester 28i combi boiler. It is about 5 years old. We are now unable to have the heating off as if we turn the heating off or the temp dial down to hot water only the pressure drops radpidly down to zero and then the warning light flashes and we have no heating or hot water.

We then have to fill the boiler and reset the warning switch. If we leave it set on ON the heating is on constantly but the pressure goes right up.

What do we need to do.