I noticed that many of the participants in the Silly Commuting Racing were using Strava so I downloaded it to my smartphone and tried it out this evening.
It claims my route is 5.1miles and I averaged 14mph with a maximum speed of nearly 36mph.
There is a nice feature which shows the times of other riders on recognised segments. On my route I completed the Lower Thames St Dash and this evening's pedal puts me 137th out of a total of 219. Not bad on a Brompton.
I always stop at red traffic lights and ride cautiously and considerately, so the average speed isn't representative but I'll try to remember to record some more commutes and aim to improve on the Dash leaderboard.