It would be logical to suspect Midnight or fae since they're the groups Norrell has pissed off most, but of course finding solid evidence to pin it on them is another thing entirely. A tower of ravens is a pretty noticeable way to go! Unless it's all contained inside the house, there would be witnesses in the street even if Childermass was sucked inside too and therefore unable to testify. If it was all contained inside the house and it disappeared quickly, there would be no immediate evidence of what had happened, Norrell and Childermass would simply have disappeared without notice.

I don't mind what option you want to go for really, you just need to decide in order to determine what the scene of the crime looks like when Lancelot and Sylvia visit and how he can proceed with the investigation.

(I've asked Shea and Frank what they think as well, happy for you guys to decide together!)