How does this planning differ from my existing mirror will?
Your existing will gifts your estate absolutely on the second death if you are married or in a civil partnership, or if you are single on your death. This means whatever you leave your beneficiaries forms part of their estate leaving it open to social impacts such as divorce etc.
What is a trust agreement?
A trust agreement is a document that spells out the rules and guidelines that you want followed for assets held in the trust.
What are the costs?
The costs very much depend on your situation, and the makeup of your estate IE do you own a business? The number of beneficiaries you have, and your priorities, once we have established this at a first meeting all costs will be discussed with you.