Ok the job is done and the floor looks good.
How i did it
disconnect the battery
Removed rear seat lower swab,pushed drivers seat full forward and wound the back of the seat forward as well.
Lifted the sill trim using a flat blade screw driver making sure not to break it or the tabs under it and then folded the carpet up and under the drivers seat.
Remover all the underlay stuff and un clipped the loom from the front seat belt point all the way round to the centre tunnel (old boy like me would call it the propshaft tunnel if it had one).
Wrap the rear lap belt round the loom and plug it in to hold the loom out of harms way.
loosen the rear wheel nuts just a little but do not remove.
Jack car up from centre member at rear and place on axle stands.(chock the front wheels first and i put the car in 1st gear if manual).make sure car is stable and safe.
Remove the rear road wheel.(this gives more light and will make it easy to see any problems later,ie fire.
Now using a hammer and a flat blade screwdriver,itapped the floor from inside to remove the hard black underseal stuff,this just broke away as most of it was brittle.
I used tin snips to cut out all the rust from the floor and to go back to sound steel to weld on.
Using a 4.5" grinder (with a guard on it)
grind back to clean metal,get rid of all the paint around the welding area,i got this as clean as i could so as to get a good weld later.
I got hold of a sheet of 1.5mm thick metal sheet,measured the hole in the floor and marked and cut out on the new metal sheet to size then i layed the new floor on 2 strips of wood and hammered the shape of the floor into the new pannel. (this worked well).
Because of the fuel tank and pipes being close to where i was going to weld i used the rest of the sheet metal i had to make a fire shield preventing sparks from hitting the tank or lines.
I placed a wet bed sheet over the back of the drivers seat,one over the door pannel and another over the centre console rear air vents to prevent any spark damage to the leather and at this point i wrapped wet rag round the loom and covered the folded carpet with rolled up wet rag as well to prevent sparks from going uner it in any gaps.
After making sure my pannel fitted and everything was as safe as i could make it,my friend lay on the floor with a bucket of water and a pump up garden sprayer on fire duty while i let the sparks fly inside the car.
I spot welded the pannel on one edge,checked fit again and when happy i welded all the way round the pannel taking special care when near the loom.
When done i ground it back a little,made sure the weld was good and nothing missed,waited 30 mins on fire watch to make sure there was no underlay or under seal smouldering anywhere (never weld and just walk away,i lost a car years ago because i did that.) and that was that part job done.
Tomorrow i will seal the floor inside and probably use P40 to fill in under the car to make the repair almost invisible.
Big thanks going out to you guys for the advice you gave me regarding the fuel tank,lines and wet rags round looms.the safety aspect worked good.Cheers all.