I got the b***d.
Not sure what the difference was today, but it wasn't even that hard. Maybe patience and another day to get the pentrating spray in. I don't have an attachment for localising the heat too well, but I did try and aim at the bottom and didn't do it for so long. Previously I was just trying to get everything hot, but is the idea to get the outer hotter than the inner?
Anyway, lots of good tips on this thread so thank you for everyone that helped.