The "best" time is arguably autumn but if you wait another month or six weeks when its a little warmer you should get good results.

I agree with Dgw - April is a good time. It is pretty mild now and as long as the temp is above 6C then the grass should germinate. If the area is prepped then I would seed it now to stop weeds getting a hold. I am trying to level off part of my lawn and will probably seed it at at start of April.