The two main watching spots outside (without standing on the side of the A27) seem to be Mill Hill and the hill up behind Lancing College.
Not done Mill Hill before, but I understand the lane might be closed so its a case of parking in the houses at the top of Shoreham and walking over the bridge up the hill.
Location is here:
https://www.google.co.uk/maps/place/...ca5644!6m1!1e1
The other spot that i've done for the last two years is up behind Lancing college. There's a car park up the top but its small (maybe 20 cars) and gets packed, although i've got in there both times with a bit of creative verge parking. There's parking in the nearby housing estate too.
Car park here:
https://www.google.co.uk/maps/@50.84.../data=!3m1!1e3
Walk to here: (5 mins)
https://www.google.co.uk/maps/@50.84.../data=!3m1!1e3
These were taken from the second spot a couple of years ago
Can be pot luck whether it chooses to loop over the hill though.