When can you tell when a FTSE 100 Company will either be going Ex-Dividend, or publishing these dates?

Answer 1

The company will usually follow a regular pattern from year to year. But when a dividend is announced the ex-div date is also published.

Most of the large companies have a schedule of dates on their website but they have to issue an official notice (RNS) giving this information.

These notices can be accessed at a number of sites. I use InvestEgate which is a free to access.