I've been complaining about this for ages, crappy HSDPA coverage in the centre of town from O2 - annoyingly just going a couple of hundred metres west or east would sort it out, so just a tiny strip of the town was missing out, GPRS was the best we could get - ouch.
Well last night at around 3:00 I checked my phone and bam a solid HSDPA signal. But this morning it had gone back to GPRS, but later in the afternoon HSDPA was back with all its speedy goodness. Sweet.