Final update – I hope.
Yesterday we received reimbursement, in full, from LNB Towbars covering the cost of repair to our van. They had already reimbursed me for the cost of swapping out the crossmember in France, and had repaired the minor damage to the Smart car. Throughout this saga LNB has been helpful, supportive and friendly. If I had any gripe, it would be that they could have paid up a little quicker. In reality, we only collected the van a couple of days before Christmas, and New Year intervened as well. I did breathe a sigh of relief when the payment came through, though.
Leigh Bryant, of LNB, told me that the faulty weld had been the result of an intermittent earth on the welding equipment. We were just unlucky to have a crossmember that was welded before the fault was diagnosed. We now have, apparently, a beefier version.