[iainn]

Thank for that very comprehensive answer. My reason for thinking it's the control unit is that the power assistance when stationary doesn't increase when I rev the engine, which I would have thought it would if the pump were worn.
I did see something on an American forum about a way to test the unit by putting 12volts across two of the terminals (can't remember the detail) so I was going to do that before ordering but when I started to look to check the price of the unit I couldn't find any listed. Hence the post on here.
I've only just had this car although I've had various Volvos in the past. I've moved to North Shropshire so don't know of any indy specialists to go to. If anyone else does that would be really helpful - I'm in Oswestry.
Is there a way to check the valve you mentioned, other than by replacing it?

Thanks again,
Iain