Ive just found a moment to throw the newly built diplexer onto the spectrum analyser to see how it actually performs. Heres where the advantages of having the matching tracking generator show themselves!
The 2m Low Pass section performs just as would be expected, cutting off at about 230MHz.
Sadly, the 70cm High Pass Filter section cuts off at 138MHz! Hmmm, I think that might be a tad too low! I'll have to have a play with it tomorrow and see what needs to be adjusted to get the cut-off to match that of the Low Pass section. Having only three components - two ceramic 4p7 caps and a air core coil, it cant be that hard to adjust!