Ok I have got to the bottom of it.
And....
I am an Idiot :/
I tried putting the sensor in, without the 'O' Rings to ensure that it didn't fit, and guess what it did!
I think last time I didn't lube the 'O' Rings and the sensor just jammed on insertion.
This time...no dramas
Sorry to bother everyone, and JR ill pick up a pre filter too.
Cheers
Dave