Is this my stopcock / stop valve?
The Polypipe parts with red and blue taps look like heating manifolds (do you have underfloor heating?) and are not likely to be the stop tap. 

You'll have to follow the two pipes to the right in the under-sink photo. One of those (the one that feeds the cold tap) will be fed from the stop tap. It will probably also connect a cold feed to the boiler and to any other sinks, bath, or shower too. 

Do you have a bath? (It could be under there).

Is your flat on the ground floor or an upper floor?

Edit to add (one of your photos wasn't loading initially)... The stop tap is the one that has been painted white, next to the earth bonding (green and yellow wire) connection. 

Also...For future reference, the black taps are on the filling loop for your boiler. If the pressure gauge shows too low pressure in the system, you'll need to let more water in to the required operating pressure.