Calling all armchair generals! Boudica's Last Stand.

[Nathan Ross]

This suggests to me that the battle might not have started until the afternoon.

Yes, that's also very possible - if the rebels did not camp somewhere close to the site the day before, they could have spent the morning on the march.

Of the two accounts, I would tend to trust Tacitus a bit more, but even he has the Romans sustaining 400 dead and 400 wounded, so it wasn't a straightforward massacre. Dio's account reads too much like a dramatised impression. The actual combat stage of ancient battles seldom lasted more than an hour or so, unless the sides were very evenly matched.