I realise it's not a Datsun but need some help on this one as running out of ideas.

I have a Nissan Micra 1995 which has a misfire. The misfire is not at idle it is while driving. It is most evident when driving at 100km/h or simular speeds down the freeway. The car does not misfire under hard acceleration. But will misfire as soon as you back off. If you clutch it, it will cough and sometimes stall.

First I pulled the plugs and they were nice and clean. The leads have tested OK with the multimeter.

Was thinking it was bad fuel so replaced fuel filter and added fuel treatment to last 1/2 of the tank. When blew old filter out it full of black liquid??? The car ran better after this but started to misfire after about 50 km.

Added about 600ml of metho to the tank today with about 1/3 of the tank full with fuel. Same again - it did misfire but it was less frequent than it initially was?

It does not have a drain on the fuel tank, so would need to rip it out to drain the fuel. Do you think this would be the next thing to try?

Or use up the last 1/4 thats in the tank now on the way to work and fill it with fresh fuel tomorrow?

it sounds like fuel from what you have said. Maybe get it really low then take a fuel line off and get the pump to squirt the rest out? then use 5l of fuel to dilute the remainer dregs and flush that out?