Evil Wolf-Cat

Sorry for the lack of posts, I've had a stinking cold and haven't been up to much.
We have noticed recently that our little tortoiseshell cat, Holly, is too scared to go out and started messing inside, till we put a litter tray down. If we shut Holly out at night she'd come back with deep scratches and bite marks on her, as if another cat had tried to kill her! Hubby also noticed a huge grey cat, which we call Wolf-cat, peering in at the windows.
Then the other night I was awoken by a cat fight on the landing (hubby amazingly slept though the dreadful wailing and hissing). As soon as the other cat heard me coming it must have legged it downstairs and out of the (broken) cat flap again. The next day hubby noticed Wolf-cat in our garden again, so it's a fair bet that he is the one who attacked Holly in her own home! Again Wolf-cat legged it when he saw hubby, he must be used to humans trying to scare him away.
Now we've boarded over the cat flap so that Wolf-cat can't get in, but now of course Holly can't get out. She is only safe outside with a human bodyguard. Such a shame, as she loves the outdoors! Horrible Wolf-cat. Not a lot we can do though...