Originally Posted by
LapLap
If you were travelling later in the day or on a different day of the week or joining the bus in a different area I'd join you in your caution, but I seriously doubt that the 6:25 bus will arrive later than 7:45am.
The website does seem to backup your view - It's currently showing 69mins from Shinagawa to Narita, at 5.30pm - Which i assume is relatively busy.
For the 6.25am departure, we have in our favour the fact it's a Sunday and that we're travelling in J, so checkin shouldn't be a major issue. (Not that i can find any minimum recommended checkin times on Cathay's website!)
Another option is staying in Narita the night before, but the last Bus leaves at 17:10, which puts us back on the train - with luggage and all the fun. Currently the cost of Bus+Hotel room is the same as the estimated Taxi fare. (Based on roughly 20000 Yen)