So I changed the fan today
For anybody who comes across this looking to see how the tank is removed...
At the rear of the black fuses/relay box, in front of the expansion tank, there are 2 clips
Press in on the middle of the clips and slide the fuse/relay box up from the bracket
Now hold the yellow lid on the tank, and pull upwards and towards the nearside wing
This releases the clips from the fan shroud as it is only clipped into place