Possible Clues:

• Each cat essentially spies a tree
• Mid-'90s Imogen Heap band
• Caught between teetotallers and half of them coming back in 20 down from the garden centre
• Whistling thorn, e.g.
• Account given to adult at intelligence agency for burning Bush possibly
• Bush expels one from a spy organisation twice
• Pod-bearing tree
• Accountant spies flowering tree
• Gum source
• Twice Bill and I get stuck together touching a gum tree