Assuming the PSU is known good, it’s likely that there is either a dead short on the motherboard on one of the rails, or there’s a blown fuse.

The motherboard led came on twice intermittently during separate times after adjusting the ATX power connector. The board also started up on shorting the power pins. However when I switched off the PSU and back on without disturbing the ATX connector, there was no light - meaning no power.

I tested the board with a few PSUs and there was still no power to the board (led not on). Had noted 2 fuses beside the power connector and near the PS2 and usb ports. Tested these and it had continuity.

Is there anyway to test the power rails on the motherboard?

You should really get a bios post code analyser card and see what codes You get.
After a long while in storage the cpu and memory sockets might be oxidised, Clean contacts where possible.
I was in the same situation, 3 socket 370 boards that either don't work at all or lock up when operating. I purchased one of those NOS
chinese boards from Aliexpress and it's rock solid stable.

Imperious wrote on Today, 00:15:

You should really get a bios post code analyser card and see what codes You get.
After a long while in storage the cpu and memory sockets might be oxidised, Clean contacts where possible.

The problem is there is no input into the board in the first place. When I connect the ATX plug to the connector and switch on the PSU, the first sign of life is the motherboard light switching on. This itself does not happen.

I also tried switching on the board without this light but there was no booting possibly up due to lack of power delivery anywhere on the board.

I would inspect the board very closely again, checking for damaged traces or loose soldier joints. Try pressing down on the chips/wiggle things, since you said the light flashed a few times. Could be a bad BGA?

If you can get some high-res images of the power area and the back, we could take a peep, too.

Did you ever replace B116?

Major Jackyl wrote on Today, 01:58:

If you can get some high-res images of the power area and the back, we could take a peep, too.

Below are the close up pics around the ATX connector:

Major Jackyl wrote on Today, 01:58:

Did you ever replace B116?

The B116 component was missing already when I got the board. However the board was working fine till recently.

Major Jackyl wrote on Today, 01:58:

Try pressing down on the chips/wiggle things, since you said the light flashed a few times.

Actually the board led lit up only twice with some adjustment to the ATX connector. However, the same did not repeat again despite multiple tries to adjust the connector.

Some more pics including the back side:

D18 is the diode which should light up first when the PSU is switched on before starting the board.

In the first picture with the whole board it seems to me that the CMOS jumper is set on 2-3 pins which means reset CMOS according to the markings on the board.
Can you check the board if is set on pins 2-3 as it appears in the picture and if this means reset CMOS?
I had a similar problem with a motherboard and I noticed later that the reason of not starting was the fact that I forgot to put set the CMOS to normal.
Once put to normal the motherboard started.

If it has not already been suggested, test the power supply itself too.

Excelsior wrote on Today, 07:25:

In the first picture with the whole board it seems to me that the CMOS jumper is set on 2-3 pins which means reset CMOS accordin […]
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In the first picture with the whole board it seems to me that the CMOS jumper is set on 2-3 pins which means reset CMOS according to the markings on the board.
Can you check the board if is set on pins 2-3 as it appears in the picture and if this means reset CMOS?
I had a similar problem with a motherboard and I noticed later that the reason of not starting was the fact that I forgot to put set the CMOS to normal.
Once put to normal the motherboard started.

This was a troubleshooting step where I put the jumper in 2-3 position for 10 seconds and then put it in normal mode while trying to power the board. But it didn't make a difference. The main issue here is the led not lighting on the board when PSU is connected.

Kittyboy wrote on Today, 07:35:

If it has not already been suggested, test the power supply itself too.

I have tried a few different PSUs but to no avail.