OK so my 7 worked... now to add a second digit, and see if the effect is OK or the project is a bust.
So I made a 6 - but this time freehand, just to see if it was possible.
Then I stuck it to the back of the 7 and lit it up.
Again, the lighter bit at the bottom is only there in the photo - the 6 lights up, but the very top bit is not as bright as I want.
I think my next try will be using the mould and bring the ending leg of the 6 back to the bottom and use a second LED.
If I don't sand the vertical return bit, then it will not emit light - and so appear to be invisible.
So far, it is looking good,