Negotiation tips? Same as anything else really. Be bold and ask what they'll throw in for you. They may well not budge on it as its already on sale and Giants aren't that hard to shift but depends on your local shop staff really. Just have to see what you can get. Pay by cash maybe a good idea. Helmet and puncture reapir kits sounds a fair enough freebie though. Gloves might be good given autumn seems to be here and don't forget some good chain lube. Finish Line Cross Country is a good one (not just for xc bikes)

Aldi is the place to go for cycling stuff. They have cheap gloves, shorts, shirts and other stuff. I bought a shirt in there the other day. Not great looking so I'll probly use it as a spare but pretty good for £2.99! Also bought a gilet which is good and a pair of shorts. Again...not best quality but alright when the others are in the wash. Less than a fiver again. If you're into it I also bought a heart rate monitor for £15. Pretty bargainous given the cheapest of the Polar ones are about £50. Does the job and has a couple of good extra features.

Good online bike stuff is probikekit.com and wiggle.com although try kelkoo and bikepimp (link posted earlier) for specifics.

Good luck!