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...that's just made me think of another question - if I start an IR with a PPL, but then get the CPL, do I then get the 5 hour reduction or do I have to start the IR first having a CPL to do the reduced-hour course?
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To get the reduction you must either hold a CPL(A) or have completed the CPL(A) course, before you start the IR.
Its all here
http://www.caa.co.uk/docs/175/Lasors_Section_E.pdf
Page 3.