Why is my Vaillant Turbomax VUW boiler pressure dropping after a service??

Hi,

I had my boiler serviced and repaired (replacement diaphragm and push rod as micro switch wasn't working correctly) and ever since the pressure has been dropping to nothing every few hours. Water is dripping out of the external overflow pipe.

Called plumber who carried out the service and repair and he said it's nothing he's done - is he right?

The pressure relief valve pipe did have a slight leak at the top and needed tightening but the problem is still there.

Could the service and repair have lead to a new problem?

Thanks in advance for your help.

Clarke