What is eating my aubergine plants

Almost certainly slugs or snails. As they come out during the night you don't often see them. Try going out late at night with a torch or looking very early in the morning while it's still damp. You can then pick them off the plant and dispose of them.
We have aubergines in our polytunnel and although I rigorously expel all snails and slugs, I found an aubergine fruit half eaten by the blighters today.
However, if you decide to use slug pellets, either use the ones which are harmless to wildlife (other than slugs) or put the metaldihyde ones under a flowerpot raised on one edge so the birds and pets can't get to them.
Birds do eat slugs and snails which have been poisoned and this can cause them real problems.