Common problem I find fitting new monoblock mixers to old cheap sinks - the hole sizes don't match up very well.
Some makes of mixer overcome this by 'belling' out at the base to make sure the hole in the sink is completely covered, but some don't.
Sometimes it's also difficult to stop the mixer moving because the base is designed for a thicker sink and the washers aren't thick enough.
Anyone seen a plate or chrome washer or adaptor that could be used?