Im after a good company that does deposit insurance.

I was also thinking of instead doing a £0 deposit and replacing it with an admin fee.

This would solve a lot of my problems. Most of my tenants are happy with this.

What are your thoughts?

Happy tenants by paying rather than getting a deposit refunded?
The main reason I take a deposit, is to encourage the tenant to end the tenancy properly (even during a fixed term). I am content if a tenant wishes to leave, and would much rather they end properly early than just abandon.
If I hold a deposit, more likely that will happen.