You really shouldn't be needing to add pectin to marmalade. I find Delia's marmalade recipe way over complicated - there are far simpler and easier ways to do things.
All I do is weigh my fruit as is, make a note of weight, and weigh out the same amount of sugar and set sugar to one side.
I cut up the fruit - basically, quarter, then slide a knife under the peel to remove the flesh, chop up the flesh (de-pipping as necessary) and wang the flesh in a pan. Then I slice up the peel (I like medium chunky) and throw that in the pan. I add enough water to just cover by about a finger, and leave that all to soak overnight.
Next day, I cook the fruit on a low simmer. You have to cook it until the peel squishes almost to mush between your fingers. One, that extracts the pectin from the pith, and two, if you don't, you will end up with tough peel.
You can add the sugar at this stage, but I prefer to leave it again overnight. And then warm up and add the sugar etc and boil up the next day. I don't watch the clock when I do this, I simply go by feel. Yes, I do the Marguerite Patten style set-test, but only once it gets to the flake stage. The reason why you do need to repeat the test to check the set is you don't want to over-boil your marmalade, else it will end up as orange-flavoured toffee. And don't forget to take the pan OFF the heat while doing the test.
This is *GUARANTEED* to set no matter what citrus fruit you use.
BTW, have had my share of red rosettes for my jams, marmalades and chutneys at local agricultural and produce shows.