Have finally got round to taking and processing another test film. Carried out the process the same (exactly as I have always done) with the same bottle of developer, but new fixer and stop. Agitation unchanged - for ten seconds (inverted) every minute.
Result: a perfect film - no streaks or marks.
Conclusion: I am pointing the finger at the Fixer - it was not a freshly made stock. I think that it was slightly exhausted and exagerated the effect with agitation. It has taught me not to take the fixer or stop for granted.
Thank you for everybodys input - it has created much debate here and made me look at the way I do things differently.
Result: a perfect film - no streaks or marks.
Conclusion: I am pointing the finger at the Fixer - it was not a freshly made stock. I think that it was slightly exhausted and exagerated the effect with agitation. It has taught me not to take the fixer or stop for granted.
Thank you for everybodys input - it has created much debate here and made me look at the way I do things differently.