Well, I managed to beat this on last night's commute. The stars, moon and tides all aligned, I had most of my gear already at work and the sun shone like a goodun' so I opted for the road bike and a bumbag to put my sandwiches in, instead of taking the hybrid with mudguards and panniers stuffed with tools, waterproofs and a change of clothes.
I pushed quite hard and was rewarded with a healthy ave of about 20.5 over the first few miles but as I got closer to work this started to slip, to I pushed a bit harder. I was about ready to collapse when I got to work but was relieved to see 20.2mph Av on the clock and a time of 31 minutes for 10.5 miles.
Return trip this morning was a new PB as I managed to do it in just a little over 33 minutes but failed to keep the average in the 20's. Final figures are- 21.30 miles, 19.8mph av, 1hr:04m:27s and this really just confirms my suspicion that the svelte road bike is hardly any quicker than the hybrid.