Help please - Suspended/Disciplinary

I have worked for Asda for 8years now and recently got into a bit of trouble with the police which resulted in me being arrested and released on bail, I brought this to works attention as am in the process of tupae over to Mitie security and wanted to know how this would impact me applying for my SIA Badge, fast forward 2 days and was asked to bring in my bail sheet which I did and was then told I was suspended pending further investigation, I have since been in and am now having to have a disciplinary for gross misconduct because I haven't disclosed what I got arrested for, as far as I'm aware I'm under no obligation to do so as wasn't in works time and wasn't in anyway representing asda. Can anybodyh point me in the right direction of where to go and also the disciplinary for gross misconduct is being done by a manager within the store I was under the impression it had to be a external manager. I feel I am being isolated and targeted as I have a voice as another guard was recently arrested and missed his shift but wasn't asked to bring in any bail papers or have any investigation.

Any help is appreciated as am looking to put in a grievance for way I've been treated.

Thank you for reading ​​​