Baby Blue Tit advice (babies might be stuck - one meter down in hollow metal fence post)

The tit family do nest in some strange places sometimes. You shouldn't interfere and in fact it's an offence to do so under the Wildlife and Countryside Act
It sounds like a decent diameter post so when the time comes to fledge they may be able to fly out. Or, unless the inside of the post is exceptionally smooth they may be able to climb up with their sharp little claws.
In any event let nature take its course.