OK So I did a bit more on the rust yesterday-
Re-coated the inner sill area with my Hammerite rust proofer/primer:
As you can see there's no rust further inside the inner sill which is lucky!
I then trimmed the new panel area to just about 1cm bigger than the original cut out area and cleaned it edges and stuff:
I also rust proofed and primered the inside of this panel area (although there was no rust on it inside anyway!):
Then I marked round the new panel and cut the bodywork to fit (with my new £15 smaller angle grinder from Argos and £1.98 cutting discs from B&Q), then fitted it roughly to check everything:
Not a bad fit, even if I do say so myself! So tomorrow I will go chat to my local garage (he restores Triumphs) as he fitted my wheel bearing for me on the cheap, and see how much he will weld it in for. I will prepare the edges for him and remove all paint for 1cm round and mask it up to stop burning etc. Then sand back the areas of primer that he needs to weld together. Anything to save a pound or two!
Then it's MOT time by the 5th when it's due.....