Thanks man,
Thats really helpful indeed as i did not have a clue.
The Sennheisers are about £100 more than the Beyerdynamic. So within my budget as i only wanted to spend between £150 - £200 max.
Not at all sure what ‘Open Monitoring’, ‘Semi Open’ and ‘Closed Dynamic’ mean ?? 
Is Beyerdynamic the same company Beyer who make microphones etc ??
So are these models specifically designed for the studio engineer who requires them for the sole purpose of Mixing/Mastering ??
Much appreciated.