Hi, so unfortunate to hear of your problem recurring. But Kelvin do give warranty for his jobs. You should bring to him again to check it out, he should be able to explain to you when gone wrong. It seems kinda fast for you to replace the strings so soon.
Anyway, the saddle is just one point of adjustment, there are others that need to be adjusted when adjusting there.
Touch wood, but could be that you just have a lemon of a guitar. Some guitars cannot be adjusted straight, that's all. An arrow straight neck which plays well without buzz is not often found. Most mid to low costs guitars will have a slightly bowed neck which is ok, as long as it plays well and little or no buzz.
Regards,
Mike
Mikemann Music Studio