I normally make the approach through the eastern entrance decks awash at night, though I've also done it at dawn and dusk, never in daylight. Always run silent even decks awash, (Those diesels make a hell of a racket but the Brits don't seem to be able to hear them too well).
When I get past the block ships I slip down to PD and make 2kts until I get into firing position. Rarely get picked up before firing unless I do something silly like broaching or leaving the scope up for too long or trying to go too fast. Never approach at higher than 32x TC either and only when I'm getting really bored with my slow progress and I've made sure the patrolling DD's and vettes are out of the way.
Fire everything you've got then run fast for a bit towards the exit, then go silent and 2kt again. Don't bother doing a reload unless you have taken out the patrols before killing the BB and merchies. Personally I rarely bother with the patrols and go for the big boys whenever I make a visit.
Als check out my Harbour rading tutorial at:
http://www.subsim.com/radioroom/showthread.php?t=96843
it was written before GWX but all the principals still apply cos the stock AI behaviour and depth of nets etc are all the same.