Hi Simon,
This is a funny coincidence. Inspired by the video posted by Chris @DL1CR, I was thinking about the many soldered connections he had on his coil and if there was no easier way of tuning. So I played around with a few coils and different materials I had at hand. Since I did not find a ferrite rod, I experimented with brass and Al pipes from Yagis and ultimately with a stainless steel spatula from my desk.
This is with the Spatula outside of the coil.
This is with the spatula inside the coil.
The range is surprisingly wide, from about 70 µH to around 100 µH. I have not driven this any further, but a threaded rod and a nut would open the path to fine tuning. (This is not new and realized several times by others).
Stay curious and continue experimenting. 
73 de Peter, DM7KN