When painting over varnish preparation is everything.
Ideally the varnish will be removed by stripping prior to painting but that is not always possible. For example, if the varnish is on staircase spindles, removal can be extremely time consuming which would of course add to the cost of having the job done.
If the varnish is not removed prior to painting it must be well sanded to give the paint grip. Otherwise the slightest scratch will remove the paint and expose the varnish underneath.
When employing a decorator to paint over the varnish ask them to quote for sanding down the varnish in preparation – in other words make sure the decorator knows that you know that that needs to be done prior to painting and are expecting the quote to include time spent on preparation.
This preparation will of course increase the cost of the job but in the long run it is worth doing as you will not have to keep touching up scratches.
However, sometimes you just want to have the cheapest job possible done. So at the very least insist upon and undercoat or one of the no-sanding undercoats that are now available.