Looks like the difference is between the rounded and unrounded back open vowels /ɑ/ and /ɒ/. This site has an IPA chart where you can hear the differences. The father-bother merger hasn’t happened in my (NE) accent, but I didn’t know that pretty much everywhere else merged the two. Interesting that cot-caught merged for NE but not father-bother.
Looks like the difference is between the rounded and unrounded back open vowels /ɑ/ and /ɒ/. This site has an IPA chart where you can hear the differences. The father-bother merger hasn’t happened in my (NE) accent, but I didn’t know that pretty much everywhere else merged the two. Interesting that cot-caught merged for NE but not father-bother.