Thanks for the offer Ken. Whatever the problem is, it lies in the grating unit. It could be a weird effect of some pattern of condensation that I haven't come across before (unlikely) but I'm more worried that something may have got between the protective glass plate covers and the blazed grating. It looks OK to the eye though but that probably doesn't mean much. The last spectrum was slightly less extended after I had very carefully & lightly cleaned the glass plate covers (for the first time ever!).
If it continues I think the best solution might be to get a new grating. I've had this one about 6 years so that's a pretty cheap annual rate for the tremendous enjoyment and interest I've had out of it! And it's not as if the unit has had an easy life. I'm not one to cosset and meticulously maintain gear (insert rough-as-guts smilie here)...
Cheers -