How can I tell if I have a leak or air in my system as my 2 week old Worcester in losing pressure everyday.?

In addition to Malcolm's answer... There is a pressure relief valve that would be piped to outside if this was relieving ie open slightly this will also drop the pressure. You can buy a tub of leak seal which you can pour into the system which will put a coating on pipe work and rads which will seal small cracks and leaks this can be brought from any plumbing suppliers.