OMCAREE - happens more than it should really but we are in quite crowded skies and occasionaly despite the checks the other a/c is not seen. The worst I had was at about 2000ft below a cloud base of about 2300 in receipt of RIS from local RAF unit when RAF piston trainer dropped through the cloud INVERTED about 200 yard away at 12 o'clock. Instinctive training takes over!