Thanks for your reply. The PC repair shop have offered to give us a new case and motherboard then move the rest components to the new case for £150. This is probably what we are going to do.
Just wondering if I can still complain to Dell. I thought a motherboard should last longer than this.
Have read on the internet about the "sale of goods act" which covers you for up to six years, that goods should be "fit for purpose" for a reasonable amount of time. Just feel so angry that they can get away with this and I am sure that I am not alone!!