A problem to deal with

After caravanning for the last 10 years in our faithful Coachman my husband and I recently found the perfect caravan 6 weeks ago with twin beds so we no longer have to make up beds every night.  The van is on dealers forecourt, brand new but we want extras fitted (motormover, electric socket, also diamond bright).  We asked dealer how long we would have to wait and he told us 4 weeks.  Perfect as we have a ferry crossing booked in August.  However, 4 weeks have been and gone and no sign of our van.  The PDI has been done but when we chased it up we were told that they were still waiting for parts!! Now we’re getting twitchy.  We sold our old van privately on the strength of the “4 weeks” so if our new van isn’t ready in time we will have to forego our holiday in France and lose the non refundable ferry crossing along with the first week campsite which has been paid for.  We didn’t even consider having a delivery date written into the contract.  Has anyone else had this problem and if so how did you deal with it?  My husband is ready to go all guns blazing to try to get things moving but I’m more for a softly softly approach thinking if we get stroppy the dealer is more likely to drag his feet!