Turns out the trip home from work is 21.5miles so today:

Cycle to work 2.5miles
Cycle home from work 21.5miles 1hr 40mins.

To add to that extra 1/2 mile it was 23degrees, 80% humidity and there was a lovely 15mph headwind. Hence the slow time. Record in previous months was 1hr 25mins so after a few weeks that's what I'll be aiming for before pushing on to 1hr 20mins. The biggest problem are the traffic lights and junctions. When not stuck at them cruising at 20+mph is possible with 36.4mph down Glassmill Lane top speed for the moment.