I have read 121 is the passmark but is this at all lenient? Or is it set in stone? Has anyone's child got in to a Buckinhamshire grammar with less than 121?
Depends what you mean by 'lenient'
Essentially, no - if you get 121 or more, qualified. 120 or less, not qualified.
However, there is a (convoluted, multi-path) review/appeals process which you can go down if you don't qualify. The closer to the magic 121, the better the chances of that being successful (strong correlation) - but sometimes a very close mark has not got in, where a lower one has.
Certainly a score in the high hundred-teens seems in the past to have had a reasonable chance, depending on HT assessment being favourable. But seemingly identical cases have had different outcomes in the past.
No guarantees, if 121 isn't reached.