Legal advisor needed

Due to one of my clients broken the agreement. I need a legal advisor to help on this issue to cover my loss.

Last year a big company ask us to produce a special products for them ( a component for a kitchen furniture ). They give me a signed letter say will purchase 1000 units, 450 pounds per unit, in 18 months ). So I supplied them the initial order for 200 units. And then 70 units, 30 units, 2 units... . By end of this month 18 months passed and they only bought about 400 units. On the start stage, they ask me keep 3 months stock in the UK for them. So when 18 months finish we still got 100 units as stock and many materials specialy made for this product.

I just wonder can I ask them to purchase all stock and materials I can not sell to others because they asked me to prepare stock and promise to purchase. How is the best procedure to force them to comply their promise? Do I need solve this issue through the court?

We are all UK limited company and my client is a major one in the industry. My aim is to cover my loss and I am not care damage the relationship with them.

Agreement and transactions details will be released to interested bidders.

Many thanks

David