Steve,
I've taken both day tours and 6-day tour with Gentle Journeys and they are excellent. I'm taking a 9-day tour with Back Roads Touring this year. Both companies have web sites and are easy to reach though email or toll-free numbers directly to London. These are small group tours that are reasonable priced. (Horticulture's tours are among the most expensive I've seen). Hope this helps.

Check out Coopersmiths, Sisley Tours and Gentle Journeys.

~Susan

I just returned from a two-week tour with Coopersmiths (based in Inverness, CA) in England and can't recommend it highly enough. We visited Stourhead, Great Dixter, Hidcote, Sissinghurst, Wisley and Saville Gardens - but I found many of the smaller private gardens we saw equally impressive. Coopersmiths' tours are small and you usually stay in one location for several nights so you aren't constantly packing and unpacking. Check out their Website.

Here is a link that might be useful: Coopersmiths One of A Kind Tours