because Postvan are no the carrier of the goods. They act as 'brokers' and arrange the transport of the goods via a third party carrier (not always the same company) which has no liability for damage in transit.
Part of the Postvan terms and conditions - which you accepted when you made the arrangements - state that in the event of goods being damaged you must notify the carrier in writing. The relevant clause says:-
"No liability shall be accepted unless the Customer notifies the Carrier in writing quoting the
consignment number of an impending claim within 14 days after termination of transit or in the case of
a non delivery claim within 14 days of the anticipated delivery date AND the Company receives a
completed claim form from the Customer within 28 days of receipt by the Customer of a claim form."
In this case "the company" means the carrier - not Postvan.
I suggest that you phone Postvan - on 01502 588 594 and tell them that you are tired of waiting. Tell them that you expect a claims form within 5 days (it's a simple matter to mail one out after all) and tell them that if you don't recieve it you'll be talking to the Tarding Standards Office. Then confirm all this in writing, and address it to the company secretary by special delivery.
The fact that you've been waiting since the beginning of the year for a claims form is nothing short of a disgrace, and speaks volumes about Postvan's attitude to their customers.