Alun and Robin,
That’s because I adjusted the micrometer hence the shift in wavelength.
I will repeat this without touching the micrometer an post pics.
I spent Sunday morning fiddling with it adjusting the collimator lens screws but the double line is still there.
When I took the belt off and went back 30 min later it had reverted to a single line !
Thanks for your inputs,
Jack
Essex UK