Looking at the forecast, I would say that Saturday afternoon would be the best bet over the weekend, and that more northerly sites would be best, i.e. Heysham, Blackpool etc.
However, I've only ever looked for Leach's Petrel on the Merseyside coast, (i.e. Hilbre, Leasowe, New Brighton, Hoylake, Formby, Crosby or Seaforth etc.) so I can't offer any advice on the more northerly locations. Personally, I'm off to take my chances at Crosby tomorrow, because it's only a 30 minute drive for me, and I saw one there today.
I don't think that the forecast could be described as good for Leach's, I would rate it as moderate. You may be lucky, but equally you may not. However, since we've hardly had a breath of wind this September, and since the Leach's season in nearly over, it's got to be worth a try if you can make it.
Good luck
Colin