Yes, this is just the kind of thing I'm looking for. Your magic has highlighted a further query problem.

The idea of the data is to capture and prove high levels of productivity. With my examples there are a large number of tasks taking short periods. With the odd task far longer, probably due to lunch or other duties. I think I need to strip these out average with a mode or median rather than the straight mean.

Still this is further towards a solution that a was earlier, so thank you very much!