If you're wishing to stay entirely spoiler free, stop reading now!
Images just posted on Twitter of filming of episode 40 showed Charles Venn, Jaye Griffiths and Azuka Oforka were still filming. This means their respective characters will not be the one to be killed off.
Amanda Mealing is also still currently filming at the studios (as of late March), and therefore has not left the cast as the character death either.
With all this information taken into account, it's now narrowed down to just 10 possible candidates for the death. Who do you think it will be? Only about 7 episodes to go now!