1. Remove start/end segments of the file
Ideally, I just want to delete the segements I don't need from the start and finish, and then resave the file. Having read the Help section, I think that i will need to save the edited version as a new file - is that correct?
If so, that will consume quite a bit of space, as they are large files. If I delete the original file, will the quality of the new file be any less than the orginal?
Yes. The easiest approach here is the simply make the first frame you want to keep as the "in" point and the last frame you want to keep as the "out" point. Then select the "Trim to Selection" option under the Edit Menu. Then select the "Save As..." File menu option to save the selected segment to a new MOV file container which you can then rename to best identify the particular segment.
Alternatively, you can mark the segement you wish to keep as above but instead select the "Copy" Edit Menu option to save the reference pointers to memory, open a "New" player under the File Menu, select/activate the new "Untitled" player, paste the copied segment to the new player using the "Command-V" keyboard shortcut, and the use either the "Save" or "Save As" option to save the new "Untitled" player as a named file.
Always try to keep 15-20% of you drive (or at least twice the source file(s) size space) free before attempting an edit. One way to cut editing space requirements is to to save the "new" file as a "Reference" file. However, this means you cannot delete the source file until such time as you finish editing the the reference file, have exported the edited file to a target compression format depending on its final use or method of distribution and have determing that no further edits are required form this source EVER.
If you use the "Save As..." option to create a new "Reference" or "Standalone" file, then the quality is the same as your source file. (I.e., this option copies the segment you want to a new MOV file container in its original compression format. On the other hand, if you export the file segment to a different compression format, then its quality will depend on the specific codec(s) and settings used for the export.
2. Edit/copy segments into new files
This is where I want to lift segments of the file and create a new file. To do this I was expecting to see the "in" and "out" markers in the timeline but when I press "i" and "o", nothing shows in the timeline (which makes it difficult to track) - is there a different method to pressing "i" and "o" to edit segments?
The basic process for "lifting" one or more segments from a source file and saving them as independent files was explained above. If not "in" or "out" points are being displayed in the timeline, then your source file probably contains a "muxed" audio/video stream of data. (I.e., the setting of QT 7 Pro "in" and "out" points is content adaptive and may not be available for certain media. In these cases you will probably be better off using the free MPEG Streamclip" app which works essentially the same as QT 7 Pro but better supports "muxed" and/or transport stream media but does not support the QT 7 Pro compositing features.)