Ta so Mike. It went better than I expected! As you suggest in your e-mail, I shall not do anything 'risky', except drive like a maniac when the car is running 100%!
Today, thanks to Rubjonny, I replaced about 200mm of signal cable in the Hall sensor wiring loom. John sent me some tips from his Junior Timer terminal tool so I could make this. My tips are from 0.5mm feeler strip and the hardest job, on the very hard steel, was drilling the holes! It works a treat! SO: Lainey now has a Mk4 GTI 25J that starts first time and has zero fault codes. With the porting I did to the head it's easy to get well 35 mpg. Before the original head was damaged we got around 32 mpg at best. Last week Lainey tried really hard and could not do better than 28 mpg even when being ultra careful. Thus the motor is 10% more efficient than from before the damage. 180 ps + 10% = 198 ps