The 1/4" is the OD (outside dia.) of the main body of the bolt.
The 20 is the no. of threads per inch. The hex head is a 6 sided head taking 7/16" wrench (Spanner?) If there is a nut, it will probably be the same as the head.
American bolts have two standard threading system. the 1/4-20 is course. the 1/4-28 is the fine threaded one. The threaded portion can be a inch or two or the entire body of the bolt.
Hope that some of this helps with the answer.
There also is the question of grades of Bolts, but thats a differant story.
Rodger a good buy for problem threads is a zeus book they are a few quid and usually invaluble.
If you are trying to source some fasteners try Model engineering suppliers as they use them up to 1/4" and sometimes above. Failing that a Landrover dealer or a vintage motorcycle dealer/repairermay be worth a try as they used them as well.
Out of curiosity where do people buy small quantities of American hex bolts/screws from? It's just the type of useful thing that Whistons' used to deal in but I've not heard of them for years and years.