Hi Ripper, this is quite a tricky puzzle. You have to study the preamble carefully. Where clues have a letter (wherever it occurs in the answer) missing when entered in the grid, the wordplay for the clue doesn't include that letter. So, for example, 2dn needs to have the "t" of "yet" removed, and the answer is MANY-EYED, but the wordplay suggests MNYEYED (YE +YE in MND (Motor Neurone Disease)), and that's how it has to be entered. So the A will be removed from all crossing answers of the first of the pairs.
And in 16dn, the "t" of "fast" has to be removed from the clue and the answer is FLASHBULB. The wordplay gives FLASHUL, an anagram of FAS HULL. Again, that's how it's entered. So the B will be removed from all crossing answers of the second of the pairs.
The originators are pretty famous writers of light opera, and guessing them will help fill the grid and possibly find the glee.
Good luck
Turnerjmw