Roof,
20a. Def. - think computer network/application
2 letter abbrev. for 'brother' around 4 letter word meaning 'disagreements' + the middle letter/heart of (sw)E(et)
5d. Def. - Like Britain
3 letter word meaning 'Decline' (I'd never have guessed they are synonyms but I checked in the dictionary) around a 4 letter Capital in reverse/revolting.