J
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As said would have thought a unvented cylinder in one of the flats needs attention,re priming or maybe over heating,requires sorting,re priming can be done by following instructions hopefully on side of cylinder,if over heating qualified unvented plumber required,the flat that is leaking should be able to tell by looking in cylinder cupboard,check visible pipe work and you should see an open plastic (usually black)pot called a tundish that shows the dripping going through it to out side
imho
they should also go to a drain or low level