Disco 2 Power Steering Fail

Hi folks, haven't been posting in a while, but here's one for ya, power steering is beyond my knowledge at the moment, hopefully not for long, but this morning driving the td5 disco it starts making a faint whine from the bay that sounds like a bus engine does, I instantly suspected it was PAS related but had a 20 mile drive home after a while it wasnt quite so bad. I had to go to work in the afternoon and it was similar but no real problems so I had it in my head that I'd have a good look into it tomorrow with the full day in the garage finishing another disco, but I went to head home this evening and suddenly I'm blessed without any power steering, and with some new slightly larger tyres on that steering was heavy! Driveable but not wonderfully safe enough I've left it near work till tomorrow.
There is now no noise at all and all belts are fine from a quick look, in only 70 miles time I'm planning a major service anyway Including PAS fluid which is very dark which I know to be bad, but other than that I'm just doing some research now on it but does anyone have any ideas and likely failures? Gonna go give it a look over tomorrow and maybe ferry it home with the RAC, many thanks as usual, from a power steering dummy!
Cheers folks

Watch the fluid in the tank with engine running, if it's not some serious activity in there then IMO the pump is gone