Please can someone advise me what might have caused this & what I should do to ensure it doesn't happen again?
The lines start up higher near the start of the film and are quiet low by the end of the film - and the lines do appear on the negative when held up to light.
poor loading of film? winding of the film? (after each picture or at very end rewind?) dust/dirt in side camera? person developing film?
Typically this C-41 35mm film is developed in an automatic film processing machine. Likely your film was developed in in a "roller transport" type machine. These machines transport the film from chemical tank to chemical tank. The film path is over and under a series of plastic rollers. These machines are highly dependent on volume and daily maintenance. If not inspected and cleaned each day, the transport rollers can become encrusted with dried chemicals and they freeze. Should this happen the film is likely damaged. The images you posted likely are the result of poor machine maintenance.
If the scratches are perfectly parallel with edges of the film, they may be caused by a grain of dirt in the camera or in the film cassette.
If the scratches are not perfectly parallel with the film edges, they were most likely made during the processing or after. Scratches in wet emulsion look differently than scratches in dry emulsion. You could try to recreate the scratches with dry and wet film and compare to the original.
I think your film was processed by hand or semi-automated equipment in a small lab. These appear to be squeegee marks from your description of them.
Automated equipment produces consistent and parallel results. You describe inconsistent and irregular results. Inconsistencies usually happen as the result of irregular, unusual, or careless manual processing.
At the end of the processing, film is usually hung up to dry in a drying cabinet if not by forced air in a fully automated film processor.
These marks are consistent with a manual means of removing extra water from the strip of film after the final rinse to shorten drying time.
Various means are used to do this. The two most popular ways are with a very soft chamois "cloth" (It's really leather) or a hand-held windshield wiper like device. If either thing is contaminated, it will scratch the film.
Now, the question remains as to what you can do to remedy the problem, if anything.
If the marks are on the very shiny side, they may be either scratches or water marks that have dried on the base of the film. If they are water marks, the film can be re-washed and re-dried or cleaned carefully with film cleaner on a wad of cotton. If they are scratches, they can be minimized with anti-scratch solution next time you get prints. Ask for custom printing.
If the marks are on the dull side, there's little you can do to save the pictures short of having them retouched to remove the imperfections if they're important enough.
