Thank you all who have helped. I have finally found the passage that I'm interested in - AIP AD 1.1.2 It says:
3.3.2 IMC Rating holder in current practice
3.3.2.1 Pilots with a valid Instrument Meteorological Conditions (IMC) Rating are recommended to add 200ft to the minimum DH/MDH, but with absolute minima of 500ft for a precision approach and 600ft for a non-precision approach.
At least I have something to print out now to pop under the nose of the CFI to prove that it hasnt changed recently, as he told me.