This is now resolved.
The points data was unusual in that it was mapped (in MI Pro) via long/lat, and not, like all my other point data, in BNG eastings and northings
So although MI Pro accept data with long/lat, and maps it, and although SSA displays it with no problem, I found that the presence of a dataset mapped via long/lat really mucked about with SSA. Queries delivered strange results and any other dataset that was in that same config (even if it was mapped with BNG) were also effected by it.
So the solution was remap the point data in MI with BNG eastings northings instead and then republish to SSA.
All is now good.