Does anyone know of a way of testing these testers, could I create a bit of
safe CO to see if I can actually get a reading on my digital readout ?

Press the test button?

That only tests the circuitry though, not the test element itself.

I would love to know a good way of testing the sensors though, I know of
people who have held them near their car exhaust but I have no idea how
this level of CO would compare with the kind of levels you would get in
a property.

I think the official way to test them is to hold a lit cigarette 50cm
away from them.