Thanks for these thoughts. To answer some of your points:
KLM and LH were the options offered by the agent the company uses.
Swiss looks good, but having tried their website the fare comes in significantly more expensive than the other two options.
The dates (14/21 February) are pretty much fixed, and we're a lot closer to London than Manchester.
So, it seems to boil down to a choice between a relatively new aircraft and the possibility of a strike at LH - unless anyone has further suggestions or views...