How can I fix it?

My Heat Store Aqua plus shower runs cold water on the hot setting. It does not heat the water no more? When it was running hot it would gurgle a lot after hald an hour or so while it was not used. While in use the hotter you wanted the shower the less preasure it was. At present though the shower sprays water but it is only warm to start and then goes cold.

John Dixon
July 2013