Hi Daniel,
So, I'm not the only one in London then
First, can I suggest you try your local adult-education college. They don't all do it, but my local one does a 'one morning a week, 5 week' course on basic PS and it's cheap as chips. I just wish I'd known about this when I started out.
Alternatively I recommend Scott Kelby's book "Adobe Photoshop CS5" - He probably does one for 'Elements' as well if that's what you're using.
It can be very tempting to wind everything up to the max in ACR to give your pics maximum punch, but everything you do has a 'noise' price and whilst it's less pronounced in ACR as in JPEG it still mounts up. Try looking at the image on the screen and decide what it needs rather than putting your size 10's on and kicking everything all over the place.
Keep it up