Air in plumbing system causing leaks in shower thermostatic valve??
 

I recently had a leak from the toilet, the plumber replaced
- the pipes going into the fresh water inlet
- the control valve system which fills the toilet water tank

After the quick fix, I noticed that the thermostatic shower valve
(something that looks like
http://www.bathstore.com/files/PRODU.../317/LARGE.jpg) started to
drip water out of the gaps between the temperature and waterflow
control lever.

I was wondering whether some air was introduced into the system whilst
the toilet leak was fixed which contributed to the problem??

Note the shower valve (from Dolphin bathroom) started to leak shortly
after it was installed a few years back and a water pressure reducer
was introduced into the system under the guarantee (it is now out of
guarantee now, unfortunately).

Right from the Mains water stop valve, the water flow to the water tap
in the bathroom, then the bath tap/shower valve before reaching the
toilet/WC. I have tried to turn off the water mains, drains remaining
system through the bath tap, shower valve, toilet (by flushing it) and
then slowly turning on the water mains, but the shower valve still
leak, despite not opening the water mains fully (and hence, reducing
the water pressure).

What could be causing the leak and if there are air in the system, how
could it be extracted from my system?