Further to a recent post about this, which responses relied on googling and people's old memories.
I have been through Euston today, so checked to get actual, current information
I can confirm that the office is still opposite Platform 17, and that it is open from 0700 to 2300 seven days a week.
It is staffed by multiple people, so there should be none of the supposed arbitrary closures of days of yore
Normally that time is quite late enough as the Lowland Sleeper ordinarily boards at 2230.
Also for Harry Potter fans the new departure boards at Euston have a section listing the next train to any station served from Euston, also a separate board giving the next train to Watford.
So it's very easy to locate the next semi fast or fast Watford train
Also at the head of each platfom is a new board giving a countdown in minutes and seconds to when the gate to that platform closes, 4O seconds before the train departs.