Written by Shakespeare
Why do they say it was co-written by Shakespeare?
Because Shakespeare wrote lots of Plays about the Kings of that time. Also a lot of Richard the Thirds lines a deliberate Shakespeare misquotes.
Let Zygons Be Zygons.
I wouldn;t know any Shakespeare lines on account of my "No-Crap-Books" policy.
The Blackadder version of Richard III qoutes the Shakespearian version several times in that first episode. Some examples:
'Now is the summer of our sweet content' (Blackadder)
'Now is the winter of our discontent' (Shakepeare)
Later just before the battle starts Richard gives a version of shakespeare's St Crispin's Day speach 'And gentlemen in London still in bed shall think themselves accursed they were not here'
Richard also uses the 'a horse a horse my kingdom for a horse' line.
Lots of Skakespearean references throughout this episode including the three old hags that meet Edmund at the end. This a clear reference to Macbeth.