My first Tachinid fly of the year: probably Macquartia grisea (Diptera: Tachinidae), a male. According to two authorities on Tachinid flies, this species does not normally appear until the end of April, so this individual was about 6 weeks early.
Ten days later I came across a female of the same species:
This was on the same bush as I had seen the male. Note the big difference in the gap between the eyes.
And this is the female from a slightly different angle:
I have to admit that on both days I had assumed that I was photographing a fairly standard Muscid fly, and I didn't spot the long spines in the abdomen until I had got home and had a chance to look at the photos on my computer.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK on 2012-03-11 (male) and 2012-03-21 (female).