Boiler overheating

We have Britony combi boiler and just recently the boiler has been venting water out of the relief pipe outside, I have to top up the pressure each day, (down to .5 bar), the radiators also occasionally make a bubbling noise and seem to be a lot hotter than normal, we also have a problem maintaining hot water to our shower, it appears to be some sort of temperature control fault, can anybody help.
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