Floppy - can't say I'm too surprised, the whole experience was a little strange. I specifically wanted to fly down the London heli lanes and cross Heathrow. We set off from Southend, and the first bit down the Thames was OK, but once we left the river he was constantly being told he was off track, and was also reading back incorrectly - to the extent that the controller said he was going to cancel his clearance over Heathrow.
Anyway, we made it overhead, had to hold between the runways while a 777 landed below us (fantastic I'd recommend it to anyone) and he was then told to cross behind the threshold - at which point he headed straight for the centre. I mentioned it to him, just before ATC hassled him about it, and he had to do a steep turn to get back on track. After that I helped him with his navigation. As I'm a PPL and he was an instructor it made me feel quite good!
Afterwards he admitted he hadn't flown the heli lanes before - I wonder what would have happened if we hadn't made it across Heathrow due to his errors? That was the whole point of the trip!