Captions for videos

We are looking for someone to create closed captions (as srt files) for promotional videos on an ongoing basis. We produce very approximately 2 or 3 videos a week. Each video is 1 or 2 minutes long. We are looking for someone to do it for £10 (or less) per minute of video.
We are looking for turnaround times ranging from 4 hours to a couple of days, depending on the video.