well to late now to advise sorry! only just seen this post, get tyre off rim check inside if there is evidence of rubber come away from inside wall usually small black balls are formed and lay on floor of tyre wall of tyre is knackered, can be used as spare but only if your ok with takeing the risk, the plum who sold you tyre in first place should have dipped the tyre in large bucket of water to check for leaks after he'd fitted your tyre and before he fitted the wheel back onto your car andwould have noticedthe valve was goosed, (its easy and cheap to replace valve.) as you say the tyre was new so you had cause to take it back to complain every time you buy a new tyre they should and usually do change the valve for new, part worns should also change but sometimes they dont bother watch tem next time