I think I've fixed it! Found a guy with the exact same symptoms on a mini forum with a HIF44. He filed off some small burrs on the flats of the needle. So I treated the needles to some love with some 400 grit wet and dry. Softened the edges of the flats along with the lead in chamfers at the top of the shoulder of the needles. I also removed the seats, and using a Q-Tip in an electric drill polished the seat with some t cut. I reset the float levels so they are flush with the body face instead of the factory 1mm gap. I've refitted them, tuned and balanced them and left it idling for a good while to see if I get the leaks. The good news is, not a sausage! Not sure which of those three things fixed it but it seems to be all working now. Not to mention running pretty smoothly.
I've driven about 20 miles with it being 30°c and she's running fine. Although she does get lumpy if stuck in traffic with this temp. Pilkie suggested extra heat insulation on the heat shield and I think he's on to something there.