There must be an easy explanation of this fault. I would not suggest you swap the RF units as this will probably introduce additional faults. Better to fault find the solvable problem [note I do not use the trendy word 'issue' that need only be talked about..] on the original unit. The superhet IF unit reflects standard radio procedures. The video output stage has very few components. I too am puzzled at the lack of response of feeding a high output audio signal to the control grid of the video amplifier valve. It has to work if the anode and screen voltages are anything near the ones quoted in the service data. Check the Z77 valve as these tend to run hot in the 1807 and literally die. I had three useless ones in one receiver. EF91's make a more reliable substitute. Hope you sort it.
[If not, uncle John has a Marconi VC53DA that badly needs that cracking tube.
] Regards, John.
PS Just a thought Mark. Is the internal audio oscillator in your E2 actually operating? I had an O/C modulation transformer in mine that created some headscratching.