Laptop No Power No Support

Our company purchased a brand new i5 8GB Vostro 3000 series laptop costing over £500, wihch after being set up soon stopped powering up altogether. The machine was sent back to Dell and according to the repair report a new motherboard was fitted (Surely they should have replaced the laptop). 8 months later and the issue has re-occurred. We contacted Dell who told us that the machine was ourt of warranty and the previous repair was only covered for 90 days. I argued the case that the laptop obviously wasn't fit for purpose from the start, to wihch they said, as a goodwill gesture they would have someone come and repair the laptop onsite for £96 max. After accepting this and giving them our details they then reneged on this offer, stating that becuse it was used by a business and not an individual they would not do it. We are now left with faulty laptop 18 months after purchase which Dell have said will cost £450 to fix. I'm absolutely apalled that a company like Dell has acted like this and our company will not be buying another Dell machine again.