The hard drive no being detected on boot sounds like a hard drive failure... but make sure all the cables are firmly attached. If the drive failed, all you can do is get a new one under warranty.
Extra fans and cooling devices should solve the temperature problems, but you have to look for external heat sources - the box sitting directly in the sunlight/near a heating duct/leaning up against the radiator/not getting any airflow/in a hot room/etc.
Whether or not you can get your money back from the compnay really depends on their refund policy and how you ask. I don't know how things work in the UK, but when I got a POS computer from Gateway a few years ago (and by the way, avoid Gateway like the Plauge - worst computer maker out there), I called up and yelled at customer service for like 8 hours (no kidding, 2 hours on hold) to no avail. When I cooled down, I called back, got a supervisor on the phone, got transfered to the customer retention department and (politely) told them that if I didn't get a refund I was going to sue them. They overnighted me all the stuff I needed to return the PC and I had my money back within a week. I don't know how it works for you Brits, but on this side of the pond, mention of lawyers and lawsuits works really well.
War is an ugly thing, but not the ugliest of things. The decayed and degraded state of moral and patriotic feeling which thinks that nothing is worth war is much worse. The person who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature, and has no chance of being free unless made or kept so by the exertions of better men than himself.
-- John Stuart Mill