Can I just mention again that this is not the same level of gamble as a GCSE remarks as (OCR Eng aside) the grade boundaries are wider.
To give an example of Edexcel history : if a student had 154 marks, they'd be 4 off an A* . They would need to drop 11 marks to go down to a B.
OCR Lang Lit, if a student had 148, they'd be 4 marks off an A. They'd need to drop 17 marks to drop to the C.
You need to be more cautious at A/A* for most subjects, but it really is worth checking how wide the grades are before writing it off.
The most I have known to be added to a student at A Level was 12 marks once; the biggest drop I have heard of was 4 marks (and did not affect grade.)
Disclaimer : just my advice! Am not accountable for any decisions!
On another note, I checked with my friend who works in admissions. She said once an offer had been firmly accepted, it cannot be withdrawn (subject to some legal smallprint) so if a remark yields a drop in grade, you don't even need to tell them, technically!