Hi Matt,
I'd be pretty sure the calibration is done in the TX head unit as they would just throw together base units and head units into boxes willy-nilly and not try to match them so closely. I have actually written an Arduino TX unit that mimics the TX head unit on the Bios WS. My idea was to have a second transmitter for my RX-Apache-Ubuntu system to listen to, and I could include graphs of other sources such as my solar hot water and photo-voltaics or the cat leaving or entering etc. It would simply picked it up as another station with my RXer. To test this out I was trying it, and getting corrupted "test" signals picked by my weather station (not good!!!).
In the end I figured out I could connect one Arduino (Rx) directly to the other Arduino (Tx) and check the Manchester encoding etc without creating any RF interference. (It was neat to have two Arduinos connected to the one Laptop at the same time, with two IDE's running and being able to debug both at the same time. Well work on the TX code, and look at what the RX was receiving on the serial monitor screen 
). To try to cut a long story a bit shorter, if you can find the point the data feeds into the 433mHz TX stage and tap that straight into the Uno (through a 1k resistor if your are not sure exactly what your tapping into 
) it could give results very quickly.
However I would suggest tapping into the official line out of the Tx Head Unit socket would be best as from looking at my board it looks as though it has some sort of line driving buffering (2-3 SMD transistors?) to drive a long line with good data. However building a simple non-inverting transistor line buffer off the 433 point would not be hard either.
My Tx Head Unit also had a couple of healthy sized diodes (like IN4004's) near the sockets which may be used to stop reverse currents into the system. I still think polarity in your direct connect cables may have been a problem.
Anyway, over to you.
Rob