I got a tip from the guy who flogs this stuff
http://stores.ebay.co.uk/AMETECH-ENGINE-RESTORE-OIL
I added a can to the engine when I serviced my Bongo last week and before I emptied the can into the engine I carefully poured out the contents into a measuring jug - BEFORE I shook the can to mix the contents.
The last 10th or so of the contents is a thick liquid with the additive and the advice was to add a TEASPOONFUL ONLY to the PS fluid.
Now - over the last few months - when the weather got a bit colder, I found my PS a little bit stiff if I was manoevering on the overrun (off throttle, autobox, revs return to idle).
When I added the fluid the steering alarmingly got worse

- almost no power until I worked the steering lock to lock a good dozen times and now the problem has disappeared and the steering is sweeter than ever. In fact, if anything the steering doesn't feel quite as (overly)light as before and there's more "feel" to it.
I'm not saying this will sort yours LEENANGE but I reckon that a worn PS pump tends to "overpower" the steering - in other words it works harder to overcome the pressure losses through wear. When the pump is required to work hard (low speed/lots of lock) the pressure to the rack can be delayed and the steering can become jerky. I know the relief valve in mines is a wee bit noisy for the first few seconds on startup.
This additive claims to restore minute scores and scratches in cylinders/pumps/turbos - I'm cynical (NOOOO I hear you say

) but remain open minded.