The fuel pump o ring number is
19B-24486-01

Thanks - I'll update with fix info soon.

Not sure why it just started leaking.

Tried putting the existing seal into hot water to revive it - nothing doing - tank still leaking.

It looks really good and as though it would still make a nice seal. However the manual recommends "new".

Ordered new one from Yamaha dealer �24

So - new seal inserted and still leaks.

I dont thing the retaining ring is pressing the seal enough. It seems to be bolted hard up before the ring is compressed.

I added some extra packing (using the old ring) and it did seal.

So not leaking.

However fuel not being delivered at a rate that will keep the bike running.

This is turning into an epic.

But I can now remove fuel tank and fuel pump in minutes.

Tried and tried - but engine will only run for 2 seconds and die.

I have put a clear hose on the output and fuel will run out by gravity. Also, with the engine on,the fuel pump pushes loads of fuel out thru the tube, so I am convinced fuel is being delivered.
However, bike will struggle to start, then only run for 2 seconds.

What are the causes for this type of fuel starvation symptom?

Is there a sensor somewhere that has died or been disturbed?

Hey, it seems you have had a none-runner for some months now. Exactly what is your bike, bike year and the current fault now as it was once leaking fuel but now isn't.

Your original problem was a leaking fuel tank but had you taken it off at one point and not fitted the couplings correctly / or damaged the plastic pressure link pipe? Have you refitted the AIS vacuum pipe, if that was dislodged?

TBH peopes would need a fresh, full run-down of what was the original problem, what you have done since, and its current symptoms compared to the ones from months ago.