I'm not totally up to speed with the ins and outs of the Sale of Goods Act, but I'm pretty sure Vodafone are entitled to attempt a repair in the first instance.
However, let's look at the practicalities. Firstly, the charger. Do you have another device you can try it with? If it doesn't work with that, it's probably faulty. You could pursue the matter with Vodafone, but you'll probably get a quicker resolution by going direct to HTC. Vodafone don't tend to keep a stock of spare parts, so would have to source a new one and that'll take time.
On the phone problem, the first thing to try would be a factory reset. That puts the phone back to its out-of-the-box state. If it goes for repair, that'll happen anyway, so you won't have lost anything, but don't forget to back up your content first. When you've done that, run the phone vanilla, with no apps added, for a few days. If it's stable, add your apps back one at a time. If the problem recurs, you've found the culprit. It's worth giving this a go as it'll mean you won't be without your phone while it's away for repair. If it doesn't fix it, you can still go down that route.
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