Hi
I would have thought the first week of June was fine? Normally I would think probably last week of May but either should do.
If you dont mind deviating off your path on the way to Wiltshire then you should be able to get to grips or have a search for the following species in Oxon/Bucks Chilterns which should be in flower:
Monkey Orchid,
Lady Orchid,
Lady x Monkey Hybrid,
Military Orchid,
Greater Butterfly Orchid,
Fly Orchid,
White Helleborine,
Birds Nest Orchid,
Early Marsh Orchid,
Southern Marsh Orchid,
Narrow Leaved Marsh Orchid,
All on Wildlife Trust reserves. A very good reason why all interested in this thread should support the (your local) wildlife trust if you dont already :t:
http://www.bbowt.org.uk/v.asp?level2id=5842&depth=1&rootid=5842
It is worth joining BBOWT for the free reserves handbook alone, which is well illustrated with decent maps, details and photos of about 80 reserves across the three counties (including all the orchids mentioned above, and more).
And Fragrant Orchid should be starting to come through on Parsonage as well.