I think that Sheila was in the throes of clinical depression certainly, and this allied to her schizophrenia would possibly have led her to scrawl notes like this in her anguish. I don't see it as a suicide note though, as it is just too vague. Also, if this was found at the scene, why hasn't it come to the defence notice before.
Apparently the Police filed it away as illegible. I do find the reference to the Police coming(or whatever the words) unsettling, though with Mike's conversation with Bamber coming to light about the "
clever bastard" remark, sussing that he might have carried out Sheila's intentions, revealing.