I only managed to get 6 units stabled but it all depends on how staggered the units are parked. If they are all parked flush then you won't get many. The ground signals get in the way of the set numbers also.
From Willesden Junction take the old oak exit, turn right and walk up to the main road. Turn left and go over the WCML. After a few minutes walk you go over the canal, the tow path is on your left. Head right and it's a couple of minutes of walk. You reach the buffer stop end were you can pick some off. Then keep walking up the path and you head round a bend roughly where the units end. You can get some more from here in between the trees. Should be easier in a couple of weeks once the leaves all drop off. When you get to the wall you can view the arrival lines to North Pole depot, and you can read off the numbers from passing units on the GWML fairly easily.
The path does look its used for fly tipping and louts drinking cheap polish beer, so I wouldn't go there once it gets dark. There was a steady stream of joggers and cyclists on it around 11:30 when I went.
