Sid1968 for a professional repair just fit a new mira shower valve
the video has had 4000+ views and added to youtube in Feb 2016
from the short video ,water could be leaking internally via a hairline crack, inside the housing,control valve could be letting by,loose fittings,water running from a different source its impossible to give an exact reason.
the video posted may be the atom without erd setup,which is significantly different to the erd version. Tbh consider fitting a new valve
good luck.
Use an Allen key to remove it from the body, apply a little bit of Loctite 55 and refit it. When the connection is removed you might want to look at the body for any damage such as a crack.
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