Well I tried the spray points but not the thick grease points suggested above, and first results are positive! The faint squeak / wheeze is still there, but as I mentioned, I think this is coming from the gaiter round the shock...

Well, the noises haven't returned in a week, even in the cold, dry conditions this morning (first 'bong' and snowflake on the dash today!) Like I say, I've not greased the springs, but have sprayed the mount from above and below. Time will tell if it's a permanent fix or just a temporary one, but for the time being, the suspension is no noisier than my wife's OEM setup - just I think noise from the gaiters, which could simply be air being pushed out...

If the gaiters are fixed both ends, so the internal volume changes when the suspension moves, a pssssh noise will almost certainly be pneumatic.