Leipzig Tax
My understanding is that tax liability is determined from where you are resident, while National Insurance Liability is determined from where you do productive (value added )work.
As many EMA based crews and all Leipzig based crews effectively start their productive work in Leipzig, the liability for German National Insurance contributions could possibly apply to all DHL pilots irrespective of notional base.
Being positioned from an East Midlands Base as a passenger to start productive work in Germany means in effect you are working from Germany and this implies a National Insurance Liability in Germany and not the UK.
Dead heading home again at the end of the week as opposed to operating an aircraft home again indicates that the real work is being done from Germany.
Might be a good time to check roster patterns and evaluate were you do most of your work from and explore how large this National Insurance Liabilty could be.