I need to replace the cam on my 3.9 classic, most suppliers advise the 4.2 cam as being the same part, some list the part No ERR5925 for the 4.2.
I've seen a cheap 4.2 camshaft.
Will it fit the 3.9 or not?

it'll fit, the main differences between cams is serp and non serp, there may be very slight differences in duration and lift between different engine sizes but it won't effect performance in an engine thats not used for racing etc.

The 3.9 and 4.2 are basically the same engine and the cams are more than likely the same, 3.5 cam will work too.

Thanks Clutchdust, you confirm what Rimmer Bros told me, but I needed a second opinion.
Rimmer Bros told me that its only the Bore size that is different, the cams are the same.
What I can't get my head around, is why there are two different parts numbers and the costs of the 4.2 cam are approx double the cost of the 3.9 cam.