Yeah, my e330 is a little finicky on focus, I did find it worked best to have a focus light and to use center-only zone. I've read a lot of people like to use continuous autofocus but I just used single - might try a dive on continuous next trip, but suspect it'll burn the battery down a lot quicker. If I wanted to have my focused element offcenter I had to do a quick halfpress and then reframe before completing the press. Shutter release thru a housing does take a bit of getting used to as well, and if you're in gloves (I'm assuming so in UK?) then you'll take a while to get the feel of it.
Not sure what lens you're looking thru now (from prior posts, the 7-14?), but keep your eyes open for a 14-54. They seem to be hitting ebay frequently now that both the 12-60 and the 14-54 mk II are out. Shouldn't have to pay more than about US$300 for it; no sense in buying new unless you think you might upgrade cameras soonish as the 12-60 and MkII both have features more aimed toward the newer Oly focusing modes, micro-4/3rds, etc. that the e330 won't really benefit as much from.