Further to the answer a very useful device for finding missing letters can be found by googling 'Chambers Word wizard'. This enables you to enter known letters with ? replacing unknown ones. It doesn't always work as in 36a as the answer is more than one word, unless the words appear together as an entry in the dictionary, nor in 16a as the answer contains a punctuation mark omitted in the grid. Obviously the more letters you know the more manageable is the list returned.
Hope this is of some use.