As pointed out by others;
If it wasn't 40 cubic metres in volume and cemented into the ground they may have been more willing to cut you some slack. They cannot base their decisions on whether you are a "nice person" or not. Don't forget that your neighbours may have complained to them directly about the store. Don't under estimate the power of jealousy!
Log stores are generally much thinner to allow air to permeate through and dry the logs so your structure would be seen immediately as a "shed". Also a tarpaulin is just as effective at keeping the tractor dry, just not as long a term solution as a "big log store".
One way of getting temporary structures converted to permanent ones are to prove that they have been there for a long time (that is only a general view of the planning law in one sentence, not a definitive answer so please everyone, don't crucify me for a very broad generalisation!) and then apply for full planning. They will have seen people turn sheds into houses this way before so they may just be trying to preempt you.