DECLAN IS POISONING CHARITY!.
I think he believes Megan, he went to see his brief about that pre nup and found out unless she is cheating there is no way to end the marriage without going skint, so he's decided to poison her. This is why shes been feeling rough the last few days AND why Declan keeps on at her to go to the doctors and get checked, so that when she drops down dead |Declan looks like the caring hubby who kept on at her to go to the docs, and knows it can be pinned on Megan whos already been cautioned for Harrassment.
this also explains why he told charity when she said dobbeh didnt want the abortion public after megan blurted it out in the pub that it was dobbehs problem now and she'd have to deal with it.
thats my theory.