This has a ‘stuck seat post’.

The seatpost is alloy, which oxidized inside the seat tube.

I’ve had the problem a few times before, where penetrating oil,

coca cola and a vice have sufficed to resolve the problem.

The seatpost crushed in the vice, and broke when twisted.

I managed to cut the back  of the seatpost,

now I will cut it from the inside, or mill it.

I cut a clear line at the back by the seatpost pin,

and managed to wedge a screwdriver in with a hammer.

I then used another screw drive like a chisel and made another split

magic!

I cannot take all the credit though,

there is a very influential bike forum

called London Fixed Gear and Single Speed (LFGSS.com), which is great resource for help or info,

where I got a lot of tips from different perspectives .

I recommend it!

(Thanks everyone !!)