Had mine done at 116Kish. As per Mark_DF05, mine was also not exhibiting any running or performance issues. I noticed nothing at all after the walnut blast.
I'm not suggesting it's a waste of money, but £325 isn't exactly small change to throw at a problem which gives no tangible benefits. I would wait personally until the injectors start to let go and then do the whole lot at once. I think the whole carbon thing is seriously over hyped in all honesty.....and a good earner for those with compressed air and a bag of nuts.