The main problems from misfueling a diesel car with petrol stem from the fact that (as Zaid points out) petrol doesn't provide the lubrication effect of diesel fuel and as a solvent it can actually inhibit the diesel that is there from providing lubrication and this can do nasty, nasty things to the fuel pump which can then have knock-on effects to other components - including small shavings of metal (called swarf) from the damage to un-lubricated pump components getting passed down to the engine which can do significant damage to the injectors in modern diesels that run more advanced injection systems such as common-rail or VW's PD engines.
Additionally there is the potential for petrol to eat away certain seals in the fuel system and possibly the fuel lines themselves.
While this all sounds pretty scary the good news for you is that these scenarios generally involve much larger concentrations of petrol in the mix which isn't the case here.
I'm also fairly certain the engine in your car will be either the XUD9 or DW8 which aren't common-rail injection engines but used Bosch mechanical injectors instead (PSA brand their common-rail engines as "HDi" and in the Xsara only the 1.4 and 2.0 diesels in the facelifted car used it). And that small an amount of petrol which has been diluted to about 3% isn't significant enough to do any damage.
If you're wanting to be extra safe or if you aren't going to be using the car much over the next few days then it would be prudent to keep it topped up with diesel to further dilute the petrol.