Only the way you are doing. But you are entitled to the information so if you keep pushing they should provide it and if they don't complain to OSCR.
All Scottish Charities have to be registered with OSCR (the Office of the Scottish Charities Regulator). You can look the details of the charity up on their website (www.oscr.org.uk) but it won't tell you the Trustees.
It does however say that the public are entitled to see the latest accounts of any charity on request direct to the charity under s23 of the Charities and Trustee Investment (Scotland) Act 2005 and the trustees are required to be listed in those.
You should contact the charity at their registered office per OSCR to ask for this. Which may or may not actually be the lawyers address.