Guys, neither i nor the aircraft I fly are cleared for Cat III ILS's approaches in poor weather. I do however calibrate them.
There is really not a lot of difference between a CAT I ILS and a CAT III in terms of equipment or how it works. The Localiser for a CAT III will generally have more elements, making it more accurate, the Glideslope antenna will be of a more accurate type and, crucially, they will calibrated to a much more accurate standard and will have a much higher level of protection on the ground. Also, it's not always possible to install a CAT 2/3 depending on surrounding terrain and a huge number of physical airfield issues.
My understanding of the difference between a CAT II approach and a CAT III is simply the inputs that are used to determine your minima. On a CAT II you will use a Radar Altimeter input (75 feet or whatever) as the minima, on an a CAT III it may be 50' (which WILL be your Threshold Crossing Height) or it may be RVR related or both. The Bollin valley at MAN causes the RA to over-read close in to the Runway therefore denying a CAT II approach. A CAT III is available because it doesn't utilise the RA, the ILS Localiser is flight checked down the length of the runway to provide rollout guidance, with the GS being checked to the 50' TCH position.
As for separate CAT III plates, unless the go-around is different for performance reasons, there is really no need for a separate plate as the ILS procedure should be the same regardless of whether you are flying a CAT I or CAT III, all the plate really needs is the Minima to use for each type of available approach......feel free to flame me if i'm wrong
Cheers