Update -
Cracked on with the new MIG and got more welding done in a couple of hours than i did in a weekend with the ARC.
Don't get me wrong, the ARC definitely has it's place in my workshop, it seems much more adept at the thicker stuff, as it was much easier to get good penetration and a solid weld with anything chunky. The thinner panel work will need the MIG breaking out though, if for nothing else than avoiding holes!!
Finished up replacing full inner offside sill, patching the insides of the footwells and a few other bits (including a length of the n/side sill that i PAID to be done last year!!)

Quick coat of primer later.....
Then came the day of the retest............... Oooooooh She Passed!!!!!
Now lets break out a celebratory can of underseal!
I now intend to cut off the filler/rust cake arches and rear doors bottoms and weld on the prefab panels from beevers
Thanks to all for your advice, and a bit from me.... anyone who is keeping their tranny going year on year, the best bit of cash you can spend would be on a Mig welder, (and some practice)
Alfie