Dishonest automotive business!

I purchased a gearbox from SDM auto-centre confident that it was a trustworthy and reputable business. I have been greatly disappointed by the fact that I have not been reimbursed what was agreed upon with the business (this was as a result of them giving me the wrong gearbox). I have been given the runaround for three months now awaiting their honouring of the agreement and to receive what is due to me.