Thanks all. Agreed, the sills aren’t ideal and weren’t done recently. I’ve jabbed at them with a screwdriver and haven’t gone through so they’re strong enough but I have no doubt they’ll be rotten underneath. The thing that’s putting me off is the sounds of how tricky the sliding door runner can be to fit and it doesn’t look like many people make good replacements. I know once I attack the sills the runner will come out as well!