Hello, Elle!
I think this is the third or fourth voucher Screwfix has sent me.
My son uses them as he is a regular customer of theirs.
11, to do (someone) can be to "con" them, "con" is an abbreviation of "confidence", or "confidence trick", and "fabulation" appears to be an American word.
I guessed it and found it in Merriam Websters Dictionary.
A "confabulation" can be a "chat".
The other one is "mishmash"/missMass.
I had quite a bit of difficulty with 1d, and not sure at all of the parse!
I had "h?h?" and hunted through Collins and found "Hahnium" which was named after a German lad called "Hahn".
I think he had something to do with nuclear fission, and was interned near Cambridge during the war.
Mind you, Elle, it took me a while to get it solved, and I'll give the parse more thought!
My other one was a new word to me.
5a, But "job" is a "post" and "doc" is "not genuine" reversed.
I think that's every day this week I have had "new words", which is good!