Update

Thanks to  the guys over on thinkpad.com

There is a generic problem with T20/21/22 known as BOD (Blink of Death)
where on boot the lights just blink and zilch. Nobody really knows the
cause but fingers get pointed at some regulator chips - Which are
either unobtainable or only available in batches of 1000s :-)

But I don't think I have that.

I have completely stripped it down and rebuilt getting rid of some dust
bunnies on the way but for a 7 year old machine it was very clean. Only
had three screws left over :-)

It no longer hangs with the screen on - just goes black but.... The HDD
is still spinning and the left hand power led blinks. It will reboot
with no problems apart from persuading it to actually power down. Lewis
in particular will recall the Intel Pro card that caused this machine
not to boot about a year ago. I noticed that when it stops the link
light on the switch goes out. At this point the uptime is in the order
of 20 minutes. I swapped the Intel with a 3Com but did not bother with
changing the driver as all concerned think it is a power/heat problem.
Dry joint or similar. This time it stayed up for around 3 hours before
it stopped but now the link light is on! So this morning I took the
card out and rebooted. So far it is still running after 7.5 hours.

Of course the questions now are:
Is it the supply to the mini PCI bay?
Are those two cards bad in some way and what would happen if they were
put in another Tpad?
Are the PCMCIA slots on the same power bus? ie I might get away with
using a WIFI card in there.
If the PCMCIA slots do not provoke the problem, are there any ethernet
cards about?

A bit more info gleaned from thinkpads.com - With a bit of plastic
hacking a T23 mobo will fit a T21 case. There are also known problems
with this board but unlike T20/21/22 they are fixable.