Sorry to spoil your sit down and hob-nob! It's too wide. I have tried.
If you don't fancy the Open Pro CDs, then go for the NoTubes 29er option and a wider 25 or 27mm road tyre. Extensive research has shown that there isn't that much difference between the rolling resistance of anything between a 21mm and 25mm tyre. Narrow isn't necessarily better. The contact patch in direct contact with the road at any one time is tiny.
The only trouble is that you might find it hard to find a wider 'racy' lightweight tyre in that width, which would defeat the object somewhat. Personally (as I have done) I would go for the Open Pro CD option. They look good because they don't have that machined braking surface, they are plenty light enough, strong, easily available and a 'proper' roady piece of kit (tried and tested over years on the road, and still used by some teams at Paris Rubaix).