Replying to Turast's post no 58 as no-one seemed to answer.
As per the preamble, the source starts with A (=1). The first clue that doesn't have extra words in for one of the descriptions is 15ac and it starts with a Q (=18). 18+1 = 19 = R so second letter of source is R.
Next clue without descriptions is 16a starts with I (=9). R(=18) + I(=9) = 27 = A. Source is now ARA and so on. You end up with the name of the series of paintings.
As mentioned this doesn't affect the grid fill but as with others I got this first which was the pdm moment for me. As someone else mentioned I spent ages thinking it was Christ's descent to Hell because of the title and because I'd got in limbo as a description, and also because it was from Easter weekend so assumed would have something to do with resurrection.