Can't really be 100% sure as the whole thing is a bit damp and covered in muck that has stuck to it but drips off the middle of the bottom where the silver section meets the black section so I guess it's coming out where they join.
It's a very slow leak - only need to top fluid up about twice a year. I'm not bothered by it but it keeps being picked up as an advisory on my MOT and has just failed on it - though I'm going to try a retest somewhere else as I think they are not being reasonable.