The 1st portion i've now found out from Heathrow to Dusseldorf is operated by Eurowings whilst the 2nd portion is a Lufthansa flight. When I booked it both were marked as Lufthansa!
Both Eurowings and Lufthansa call centres haven't filled me with confidence and neither seemed really sure of how the connections work at Dusseldorf.
If anyone has experiences of connecting between the two i'd love to hear it. I've managed to ascertain that the luggage should be checked through from Heathrow to Newark without needing baggage reclaim and that both flights *should use Terminal A in Dusseldorf. However the Schengen process there seems a bit vague (I'm a British passport holder btw) and neither helpdesk could confirm what passport/security checks I needed to go through. My transfer time is 60 minutes.
I'm really looking for a Dusseldorf airport expert please!