I would dearly like to mount my lambda sensor in the engine bay. However my exhaust downpipes do not collect together until they are outside the engine bay. I could mount the sensor into a pipe that contains two of the outlets. Would it be stupid idea to do this or could I get away with it?

Are you suggesting having a cross-over type tube between the downpipes and putting the lamba sensor in that?

I'd worry about the "air" inside the tube not accurately reflecting what was going on inside the downpipes. However, I think that's me being silly. In practice it would probably work fine, the downpipes would be pressurised at different times so the pipe would be flushed through quite regularly.

It is a 4 : 2 : 1 type of manifold and I was thinking of puting into one of the '2' sections.

I would do it properly. A narrowband sensor is already a compromise, why water it down even further.

With only 2 cylinders feeding, each will have a have 50% effect, with all 4 they will only have 25% effect.

See my maths are very good

I personally would (well I did) feed it through the side panel to fit from the inside if the pipe so it is protected. It actaully looks quite effective.