I've really struggled to find anybody this summer. I've used Soul Gardeners in the past (very good) but they are pretty much booked up 3 months in advance these days.
Most gardening companies seem to want a regular contract and not a one off deal.
However I've recently used the Tooting based Anna's Gardens and thought they were very good and competitively priced. A little flaky to deal with but the work is of good standard