I have been mixing and matching chips as I suppose everyone does (making
sure that they are all, e.g. 100 mhz chips) and have suddenly started to get
a supposedly important error message telling me that the SPD is missing or
inconclusive on some of them

I could probably tell by elimination which ones are giving the error message
although at the moment it looks as if all of them are and some will let me
go into windows, some wont

Is there any test I can run to tell me if a chip will work OK even with this
error message and what is wrong with others or what is wrong with them in
this particular system?

I have tried memtest but it runs interminably and SEEMS to end up just
telling me if a chip is bad, which I suspect none of these are.