Hello I am a solicitor with over 15 years experience. I will try to help you with this.
Is you deposit in a guarantee scheme?
Yes it is called - My Deposit
As far as the deposit is concerned you will probably be dealing with this already through the My Deposit scheme and from what you say it seems likely that you will prevail.
As far as the over payment is concerned this is your money that you are entitled to have returned. You should write to them explaining what has happened and demanding your money back. You should tell them that you want the money back in full within a set period (eg 14 days). Warn them that you will start proceedings in the small claims court for recovery of this money if he does not return it to you within the period. You could tell him that you regard his actions in holding onto your money as theft and that if he does not return your money you may have to inform the police of the theft.
If you follow this link you will see the government site about making a small claim which is very cleat and he https://www.gov.uk/make-court-claim-for-money/overviewlpful.