i've read that a vinegar/water mix is fine for cleaning the cage and bowls, is this correct?
Yes, you can do that; I actually use just straight vinegar for when I clean their bowls. The use of dishwashing soap and bleach is from what I've read, not reccomended. I have to admit to using this sometimes, just because the grime wouldn't come off with the vinegar. Basically you should only use it if you see the need, and like you mentioned; thoroughly rinsed off.
i've also noticed there are a plethora of bird suppliments for sale, are all these necessary?
Not too many of them are, actually. I've read on several websites that an over-accumulation of supplements can actually cause problems in birds, mostly pertaining to the liver.
are there certain ones that are a 'must have'?
Absolutely. Molting is a stressful time for birds, so stock up on some molting supplement. That can be picked up at your local avian store, pet store, and rarely; your local supermarket. This contains some very important vitamins and minrals needed to get through theis stressful time. It's also a versatile supplement, and it can be used during other stressful times (moving, etc.).
Also, stock up on cuttlebone. It's important that your bird gets enough calcium, and that he keeps his beak in good shape. And when I say stock up, I mean stock up! This stuff goes amazingly fast.
i actually haven't taken a good look at bird foods in the supermarket, but is it possible to get a decent mix from a supermarket?
is it better to look for one at a petshop instead?
I rarely find anything good at my local supermarket. I wouldn't suggest going there to get your bird's food; rather, go to a pet store or a store specifically for birds.