The 1.4x TCs are compatible with Canon EF lenses that have a maximum aperture of f/5.6, provided they are 7-contact lenses which most of them are - apart from some L-series which have 10 contacts.
The 2x TCs are compatible with EF lenses f/4 and faster, again provided they have 7 contacts.
With 10-contact lenses, the apertures are f/4 and f/2.8 for the 1.4x and 2x TCs, although it is possible to fool them by taping over three of the contacts!
This means that you should be able to use a 1.4x TC with your 75-300 f/5.6, but not a 2x if you want to retain AF. If you're willing to focus manually it doesn't matter, but manual focus isn't easy with a standard viewfinder and a fairly dark image. Remember that a 1.4x TC will reduce the maximum aperture by 1 stop, and a 2x TC by 2 stops.
The Canon TCs are very expensive and really no better than the Kenko Pro300. If you do an eBay search you'll find at least two HK companies selling the Kenko Pro300 for around £85 + postage. Make sure you get the Pro300 version though - they do a cheaper one which isn't as good. (BTW, I believe that Steve has one of these.)
Hope that helps!
