boiler pressure water heating baxi combi 80e

our boiler pressure is dropping to 0 even after having the blender valve replaced, paid £80. Maybe 2weeks ago. This was when our pressure was at 0 and only cold water was coming out. Now my water is going hot and cold every 15/20 seconds from the bath tap. Any ideas only moved in a month ago

Hi all, First time touching anything boiler related but I’ve been asked by my landlord to increase the boiler pressure after I informed it was at 0 and no hot water, he sent a YouTube link for a completely different boiler so I’m not sure how to apply it to my own, I’m sure it’s this silver...