The 35 and 35a come in 10min intervals as do the 20 and 20a (I havent got access to timetables at the mo) - but hypothetically:
35 - 00
20 - 05
35a - 10
20a - 15
35 - 20
20 - 25
35a - 30
20a - 35 and so on.
Whether they are evenly spaced out like that in the real timetables or whether they are evenly spaced out in real life is another matter...
You are right though, it has the potential to get very busy.
RE: Go North East