Hi Death Ball,
Once again, many thanks for your help I will let you know what happens.
Changing the number teams in a league shouldn't affect the working of the league has enough teams to fill promotions and relegations. So setting 24 should have no problem.
To update the FA Cup:
Go to nation rules.
In bottom left window, to the cups.
Select FA Cup and remove it.
Click add, choose FA Cup (winner qualifies etc). Next you'll have to choose competition, pick FA Cup. Then in select teams, add all the divisions you want to have its teams playing it. That's all.
When you try this save in a new file, because I've never fiddled with cups. It should be what I've told you; but better be safe. I know this caution would be taken by you without saying, but just making sure.