The boiler is an alpha cb24x. Reason to flush the system. The water is black plus losing pressure possibly due to muck on prv seat?? No hot water except when heating on.
MMM. don't think your only problem is the muck and on another point do you really want all the rubish from the system in the boiler, only way to do this is a powerflush, chemical fill and drain or mains flush and get someone to look at your diverter valve would guess as you get HW with heating you need new diaphram.
with the system this far gone, you need a reliable RGI to sort it out.
chances are that what you are aware of is only tip of iceberg.
sounds like your diverter needs cleaning/replacing, that means your plate exchanger is probably also clogged up as are pipes and radiators.
powerflush would be a good start as this may solve 3 problems in one go, if you are lucky.
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