Thanks for the reply Steve. To close off this thread. I eventually figured out the issue - long story short you need to remove the manifold and the air filter housing, or as I did , mostly remove it and squeeze your hand/arm in underneath. After many hours and frustration I can confirm that while it is possible to get a new dipstick tube loosely in place from under the car, I found it impossible to securely push it into place and to also fit the top rubber bung that sits on top of the tube and under the inlet manifold.
As a side note specific to this car - I eventually worked out why I could not see the lower metal tube for the dipstick from the top of the engine, someone had previously inserted a bung into the tube so that debris could not get it. While this was sort of positive news I can’t help but wonder why they didn’t just put a new tube on while there were in there, especially considering it was about £8… Happily, after all of the above the oil level was absolutely fine.