Reading it properly...
And the definition of an "Alternate Base" as described in CAP371
This is designed to allow crews to report at an alternate base allowed within an are allowing 90 mins travelling time to both the main base of employment and that alternate base.
Seems to suggest that you can be 90 minutes from the main base AND from the alternate base, meaning a 3 hour distance between them.
But still, depending on when duty starts trying to get to Brum or Man within 90 minutes, even from the local area, would be hopeful!