According to the Network Rail WTT Timings it's a bit sparse:
2H99 1640 Cambridge to London Liverpool Street
2H57 1851 Cambridge to London Liverpool Street
Then nothing until:
2H73 2251 Cambridge to London Liverpool Street
I really have no clue how reliable it is as to what will turn up though.
Unless there's major service disruption, they're very good at keeping to their diagrams.
It's 1H99, 1640 from Cambridge (calls at Audley End, Tottenham & Liv St) that's a 12 car 379.
The reason the 379's are so few and far between on Cambridge services in the evening is that out of the 30 units, 9 are allocated for Cambridge, (18 for Stansted and 3 Maintenance/Spare). There's 3 12 coach trains for Cambridge in the evening peak- 1707 Kings Lynn that comes from Orient Way, 1737 Cambridge that runs empty from Cambridge - London, and 1807 Kings Lynn which comes off 1H99.
The 1851 from Cambridge back to London is the rear 8 coaches off the 1707 from London (when the front 4 comes off and carries on to Kings Lynn).