Almost completed, just three incomplete answers. I've been thinking letters into spaces, but so far have not come up with answers which fit the clues.
Hints would be, as always, much appreciated.
27 across: Diana shortly needing European aerial (6)
Letters: DI - O -E
The word DIPOLE fits, but no idea why!
18 across: Room occupied by old bag (7)
Letters: - O - D A - -
27 down: Communist addenda for travellers (6)
Letters: R - D - R -
If the first part is RED, then I'm stuck! If it's I then riDERS fits, but cannot see why.