Tliak that wasn't a plumber you had in it was a F*$£ING idiot.
Any decent plumber would have had a look in the tank, if water was in it why change a ballcock (extra bucks).
You have an air lock.
The pipe to the kitchen probably comes off first and the bathroom probnably off a long run across the attic.
Think of it this way. The pipe has to rise (or at least be level) from the tap to the top connection on the cylinder (not stricktly true but trying to put it over in laymans terms). Your photos show the wrong pipes (heating). You are only concerned with the pipe that comes off the top of the cylinder.
Pipes can get bent by people crawling over them or they might never have been put in right in the first place.
Check for dips and RISES in the pipe to the bathroom (follow it) and fix them. Copper bends easily
Check the size of pipe that is coming from the black tank to the cylinder. With both being in the loft the one from the black tank to the cylinder should be bigger. If it is the same size it WILL draw air in.
BTW get a decent registered plumber not a course cowboy