Well I've discovered the reason I heard nowt at all on 23cm yesterday.
I'm using an old LMW transverter and the 2nd RF stage of the RX converter hoots (I think the 1st stage might be dead altogether) Looking at the layout its not that surprising - there is no shielding around the individual stages and not many ground stitching vias, so I would expect it to lack some stability.
Despite efforts today to add some shielding made from bits of PCB I haven't yet succeeded in taming it. This is a really old design now though (think I built it about 25+ years ago), which has me wandering if I should look at an alternative, more recent approach and just use the remaining parts that work OK (the LO chain, PA and switching all work quite nicely)
The RX conv in its present state would make quite a useful test source for 13cm & 9cm when I eventually get going on those bands.