Water pressure keeps increasing?

symptoms are;

bar rising from 1.0 - 3.0+
filling loop removed.
draining rads of approx 2-3 litres of water every 8 hours.
turned mains water off and pressure stops increasing.

Advice from this site said it was my Heat Exchanger.

Had a plumber out, he contacted the Worcester Bosch helpline and has come back and said i need a seperate expansion vessell to be added above my existing boiler.

surely an expansion vessell is to release excess air in the system? but the problem seems to be some kind of water leak from my mains into my central heating system.

I am going to get another plumber out for a 2nd opinion, should i stick to my guns and get him to replace the heat exchanger or will a 2nd expansion vessell do the trick?

worried i will spend money out on a 2nd expansion vessell and that won't cure the problem?