Thank you both for your replies, that's exactly what I did! I bought sone 100mm i.d. plastic pipe (Toolstation item '100 Round Pipe 100mm x 350mm') (i.d. means "inside diameter", I hadn't realised this when looking up extractor fans) and cut it so that it was about two inches longer than the fan cylinder, slid it over it, then fitted a plastic 100mm pipe connector to the top (Toolstation item 'Straight Round Pipe Connector 100mm'). This meant that I then had an inch or so of 100mm outside diameter connector on which I could fit the very tight inner ducting (of the insulated ducting.) Then I put a cable tie around the inner ducting, and pulled the outer ducting and insulation down to meet the loft insulation, which was all around the pipe cylinder.
To finally check everything was working correctly, I turned the fan on and held up an orange smoke pellet, and saw all the smoke get sucked up into the fan, and come out of the vent in the soffit. I would like to try this again with a helper holding the smoke pellet under the fan while I am in the loft, checking for any leaks up there.
Thanks for your help!