Sorry, I was meant to include the following;-
Your ex will probably get a Community Punishment Order if found guilty and a Bind-Over to keep the peace. Hopefully the police are also pursuing any driving offences and hopefully he 'resisted arrest'.
Yes you may have to attend court as a 'Key Witness' to give evidence. The police and / or the Witness Liaison Unit will advise accordingly.
The other girl should request that the police warn you not to contact her again under the Protection of Harassment Act then this may deter any future problems of this nature and hopefully put an end to any further aviodable nonsense.