Quote from Grahame "We also have the problem of the handprint on her night dress. Was it Sheila's or not? If it was Sheila's then her hands could not posibly have been spotlessly clean."
If the swabs taken from Sheila's hands did not show blood, if tested for blood, then as you point out the hand print on Sheila's night dress would indicate that it belonged to a third party. To make such a hand print there must have been some fresh blood contact with the third party hand and then Sheila's nightdress.