From where is it dripping exactly? Could be the pressure relief valve.
Look at this site for help and spares:
https://www.showerdoc.com/mira-event-thermostatic-post-1993-shower-spares
Could be one of several types:
Mira Event (Manual) - Post 1993
Mira Event (Manual) - Post 1996
Mira Event (Thermostatic) - Post 1993
Mira Event (Thermostatic) - Post 1996
Mira Event Xs Manual
Mira Event Xs Thermostatic
Find yours, click through and scroll down the page to find frequently asked questions.
acp, March 2017