Access to experts in the identification and dating of pottery is essential for back garden archaeology. There are books which can be studied and used as a reference (like "Pottery in Britain" by Lloyd Laing which has been invaluable) but it is easy to jump to conclusions, especially if it helps to push a date back in time.
Another expert who would be handy is a geologist. You get used to the common forms of naturally occuring rock (mostly chalk and flint here) and just toss them to oneside. But sometimes a piece stands out as unusual and just has to be kept. An example from the current dig is shown in the photograph. It is heavy for its size and it has a cubic, crystalline composition. I thought that it might be metallic but it doesn't give any reading under a metal detector. I am glad I kept it and I'd like to know more about it.
In the course of excavating the ditch feature I have found many cracked, formerly smooth, stones. They stood out as uncommon from the start i.e. you don't find many uncracked, smooth stones. As they accumulated it became clear that they were "pot boilers", stones that were heated up in a fire in order to be dropped into a liquid for cooking purposes although some experts say the main use was to roast things. Either way, the heating and cooling process caused them to crack and they must have been discarded. I could easily have ignored these stones and regretted it.
If it is unusual in any way, keep it!