A female soldier fly, probably Sargus bipunctatus (Diptera: Stratiomyidae). Sargus flavipes is similar looking but smaller (6-10mm against 12-13mm; this specimen was definitely longer than 10mm), and comes out earlier in the year (June - September against July - November; this specimen was found in late October).
This photo shows the metallic blue-green abdomen with the orange patches just visible on the first 2 segments. (The "black abdomen" in Stubbs and Drake might just be a result of them looking at faded museum specimens.)
This photo shows the two white spots (bipunctatus) between the eyes:
Photos taken in the Wilderness, Whiteknights Park, Reading University grounds, Reading, UK, on 2011-10-22.