I am distantly related to the Forbes family but don't know their family tree well. However, I have some references that might help you: 1. Memorials of the family Forbes of Forbesland by Alexander Forbes, Aberdeen 1905. 2. Memoranda relating to the family of Forbes of Waterton by John Forbes (b.1754) 1857. 3. The genealogy of the House of Tolquhon by J. Davidson, Aberdeen, Constitutional Office 1839. 4. Memoirs of the Banking House by Sir William Forbes of Pitsligo 1860. 5. The Genealogy of the Family of Forbes by Matthew Lumsden of Tillicarn 1580 (?) ed. by William Forbes, Inverness 1819, reprinted 1883.
If you want some suggestions about other reference systems in the U.K. please let me know.