Power steering pump
Where in the UK are you? my mate has lots of those

I agree with Royce. 99% certain that it's the p/s pump.
I assume you've checked the reservoir fluid level.
If it gets too low you get the symptons you describe.
A top up cures it, with no harm done. Then look for the leak.
Mike.

Others have mentioned the power steering pump as well as the fluid
level - but you should also check for air in the system.

If the fluid level was low then it could have leaked out or maybe a
mechanic did not top off and/or bleed the system properly when the
steering system was last worked on.  Determine if you have a leak.

If you don't have a leak and the fluid level was low then air probably
also got in the system, defeating the hydraulics and will introduce
vibrations (humming).  You can refill the resevoir but if air has got
in the system unless you bleed it, the air will remain and defeat the
hydraulics.  Also as the air exits via the resevoir the level will
fall again.

So essentially you must check for air and bleed it out of the system
before "you truly finish the job".  To work the air bubbles out you
refill to MAX level, start the engine and work the steering wheel back
and forth, checking the fluid level and refilling as it falls.
Continue till the level is constant and you notice no more bubbles.
Then refill to MAX level and recap the resevoir.  It would be easier
to work the steering wheel if you have the front of the car on jack
stands.

Make sure you use the proper fluid recommended in the owner's manual.
If the system works fine from there on you should keep an eye on the
level for a few weeks in case you do have a leak.

Michael