Yes, two of the four northbound were through from Cambridge.
Don't know if you could overtake them via the ECML/Peterborough!
From memory, you could (even pre HST) often do better via Peterborough, depending on connections.
I remember on an enquiry clerks' course around 1980 being set Hull at 0900 to somewhere beyond Ely. The expected answer was change at Donny onto the said 1029 DMU and again at Ely. Cleverclogs pointed out that you could stay on the Hull-London and change once at Peterborough for exactly the same arrival time (thus missing 3 hours on a Cravens...) - the course leader didn't seem pleased
The first northbound from Ely in those days connected off the Continental which was by then running via Nottingham - it went to Sheffield via Worksop.