I suspect that due to your frustration you are going to end up badly damaging the watch.
Firstly do NOT lay the watch in the palm of your hand and try opening the back by pushing down and turning. Hold the watch by the lugs and very very gently grip the side if back cover to turn. Pressing down just forces the back tighter. It should be a gentle grip on the sides so as not to tighten the back (squeezing) it into the case.
I doubt any jeweler would charge to open it, but for 10 or 15 £ its better than your frustration