How can I stop the shower head dripping?

Hi, I am hoping you can offer some advice. We have very recently had a complete bathroom replacement which appears to have been carried out by John Wayne's brother. Although it actually doesn't look too bad, there are a number of issues (including the fact he fitted lighting unsuitable for a bathroom which is going to cost £200+ to correct), and where possible I would like to resolve without added expense.
We have a combi boiler and because of this Bathstore (who supplied the fixtures and fittings), indicated that we could operate the new shower from our normal hot and cold water system. Accordingly, this was done using a thermostatic bar valve. Sorry have no idea of the make as because our "plumber"
didn't like what Bathstore supplied he obtained another and we sent the original back.
Before he left he showed us our new bathroom but we couldn't try anything as the place was coated in mastic "which needed 48 hours to dry" according to him. So, 2 days later we attempted to turn on the shower to find the knob extremely stiff. Eventually it did move and we had water, but we clearly cannot turn it off to the same degree as he'd left it and in consequence it constantly drips from the showerhead. We are reluctant to attempt to undo any of the nuts as the water temperature is also controlled from this valve, but the drip is really annoying, especially in
a brand new bathroom.
Thanks in anticipation