I am 70 to 80cm down and shells like this have started to appear. I am encouraged to find them as they mean that the soil I am going through was on the surface at one time. Either the snails were part of infill or they got into the ditch as it was filling up. Judging by the nature of the soil it seems like the former. In previous digs I have seen the layers that have built up in the ditch as fine dark soil. What I am going through is very coarse with a lot of chalk and flint so it could be late Iron Age infill.