Wish I had but unfortunately I gave my mechanic cash to collect the part for me, I do have a receipt though

I had a problem with my Renault Clio which turned out to be a faulty brake calliper. I purchased a reconditioned calliper from UNI-PART which was around £150 and was fitted by a friend who is a retired mechanic. This was 4 weeks ago. The car started showing signs of brake problems again after 3 weeks, which I noticed as the brakes set on fire after only travelling 1 mile. Luckily I didn't have my children in the car and just happened to stop at a shop when I noticed the fire. as my friend was ill I took the car to a local garage who stripped the brakes down and found my reconditioned calliper is again faulty and had caused the brakes to be sticking on hence the fire. The garage contacted UNI-PART who advised as the calliper had been paid for with cash I would have to pay another £150 for another calliper and they would send my faulty one away for testing which they would refund if they deemed it faulty. I cannot afford another £150 and do not think they will admit to the fault, plus once they have the part I have no proof at all. Please help as I don't know what I can do. I was under the impression the part would be covered by a warranty and if there was a problem with it they would give me another. Plus there's the added safety concern that if I hadn't stopped at the shop would the fire have spread and the whole car caught fire???? I haven't spoken with them myself as the office is now closed but I wanted an understanding of where I am legally before I contact them. Any advice will be greatly appreciated.