24a - Aren't there 2 possible entries for this - a foreign letter and a rarely used (but in Chambers) word for "to lessen" - in fact, the word has 4 separate definitions in Chambers.
Another comment. The two characters referred to in 1d appeared in the public domain long after I left the UK (30 years ago). However, because their abbreviated names are common 3-letter combinations, they feature quite frequently in word plays.