"...My phone often has a skew of rather more than that on its clock and yet can get a decent satellite only location fix via GPS"
Light moves at about 300000 km/s. One millisecond corresponds to about 300 km, so I can guarantee you that the GPS circuitry in your phone has the time nailed down to better than a microsecond (300 meters). But I do know what you mean. I've seen devices where the time displayed clearly wasn't coming from the GPS.
On at least one older, stand-alone GPS unit, you could turn it on and watch the clock as it searched around for satellites. When it got a position fix, the clock would jump by a few seconds. It had a somewhat crummy quartz clock, just good enough to figure out which satellites might be visible. Once it could "see" four satellites, it had four equations and could solve for four parameters (x, y, z spatial coordinates and delta-T clock error). Then it would adjust the clock accordingly. (Four satellites are enough -- or three, plus a clock so you can set delta-T=0 -- but additional satellites do help you to reduce errors and give you some redundancy when you lose a satellite or two behind a building.)