Thanks. Were they truly on the second row? Theoretically they should have been seated next to Charles and Camilla based on the order of precedence that is nowadays applied. Next to Timothy, the only person who would normally be seated over there (i.e., in between the Duke and Duchess of Gloucester and Prince and Princess Michael of Kent in terms of precedence) would be the Duke of Kent. Normally, if princess Alexandra would have been present, she would theoretically be seated behind the Duke and Duchess of Gloucester (unless she 'accompanied' her older brother (and therefore was seated next to him)) - next to (but lower in order of precedence) her younger brother.
The order of precedence and the seating arrangement would normally be (with the queen not present):
2 - 1 - 3
5 - 4 - 6
8 - 7 - X
1: Prince of Wales & Duchess of Cornwall
2: The Duke & Duchess of Cambridge
3: The Earl & Countess of Wessex
4: The Princess Royal & Sir Timothy Laurence
5: The Duke & Duchess of Gloucester
6: The Duke of Kent
7: Prince and Princess Michael of Kent
8: Princess Alexandra