Car Hire and Damage Waiver Crisis!

Hi
I have been using MSE for some months with great success but this is my first post. It is a long and miserable story so bear with me. In June my family and I travelled from the West Country to Durham to see our son graduate. However what should have been a lovely day was completely ruined. We had flow up as my elderly parents were with us and it is a 6 hour drive. We were then going to hire a people carrier for a couple of days to do all the celerating and then to travel home with all our son's possessions. On the morning of the graduation we were picked up from our hotel by the car hire company and taken to pick up the car. When we signed the forms we were asked if we wanted to pay an extra £28.00 to cover the £500 Excess that would have been payable otherwise for any damage. We said yes, signed the forms and paid for the hire plus £100.00 deposit. My husband drove us all up to the cathedral in plenty of time and went to park the car. Although there seemed to be spaces at the cathedral they were only for staff and even disabled people had to be dropped off. ( My mother is disabled). Anyway I queued for about 30 minutes to get in and then hubby returned looking very flustered. I asked what was wrong and he said that the car had been written off by a parking bollard. He had had to leave the car on the side of the road and ran all the way to the cathedral.
We went into the ceremony , which was great , and when our family came out we had to tell them what had happened. When my husband had driven back through the market place he followed another car out which paid for a ticket at the machine and as he had not parked he presumed he did not have to pay. However as he drove out a parking bollard came up out of the ground and pierced the engine of the car. He said it actually lifted the car off the ground. When he got out there was oil all over the ground and the bollard was stuck into the underside of the car. The bollard then went back into the ground. He just pulled the car to one side and left it to get to the ceremony.
Anyway after we had told everybody we had to try to get a taxi to the drinks reception which was nearly impossible until a kind stranger took my parents and son and daughter on up. My husband had gone to see to the car and at that point the Rac Recovery man arrived ( I had called the car hire and the RAC just before we went in to the ceremony). He took me to find my husband who was now at the police station as the car had been towed away!
We spent 2 hours getting it back and the Rac driver then towed it to our hotel to be collected the next morning.]
We eventually got to the reception 5 minutes before it ended and had not been able to enjoy it at all! When I spoke to the car hire company in the morning about organising another car for us they dropped a bombshell when they said we would have to pay for all the damage as is was my husbands fault. However we eventually got another car and drove home and when we dropped it at our local depot I asked about the damage and was told the insurance company would deal with it. About a week later a letter arrived from the Hire company saying the damage was £4500.00 plus and we would have to pay. I did a little investigating on the net and found that it was in fact a bollard for a congestion charging zone which Durham and London are the only places to have this. Because my husband had not paid £2.00 the bollard had been activated and damaged the car. We also found out that this bollard has damaged many cars including a police car and a lorry. We were also told that it is in fact an anti terrorist bollard not just a parking one and ascends with a force of 13000psi. ( VERY FAST! ).

Anyway sorry to ramble but this really is getting to me as the car hire company not only want £6500.00 ( to include loss of earnings, loss of value on the car plus the repairs ) but also kept my £100.00 deposit and then debited my credit card account with an extra £400.00 thereby taking the £500.00 excess payment even though we paid the £28.00 to cover this!
Also a week after the accident my husband tried to commit suicide ( he is suffering from extreme depression and is having a lot of problems with his job ) and he was only saved by our son's quick thinking. So while I am trying to put our lives back together ( and luckily things are going well although my son is having to have some counselling to help him) and deal with all our other debts ( of which there are many ) the car hire company are constantly phoning me and threatening me. But this week I finally had the time to go to the Citizens Advice who wrote them a letter telling them they were in breach of contract.
Anyway sorry for the ramble but can anyone tell me if the hire company can do this or are they just trying it on. Surely they shouldn't hhire out a car if the insurance and damage waiver are worthless. Please can anybody give me some advice. The car hire company is Enterprise Rent a Car by the way!