If the company relies on your product to make money and they aren't paying you that makes me wonder if they're actually making money at all. A customer that doesn't pay ceases being a customer and becomes a liability, and if they're not paying you for services already provided then hanging on hoping they'll pay you for future provision of your services is probably futile.
Personally, for that kind of amount of money combined with the delay in payment I'd be seeking legal advice on (1) recovering the debt and (2) whether the "customer" has defaulted to the point the contract is breached and I can pull whatever support and other facilities the contract commits me to. Going through the courts may or may not be required in the end, but a stiff letter on legal letterhead might focus their attention, as might taking away your services and their access to your product if it will hit their income and your legal adviser's opinion is they are unlikely to win a case if they try to hold you to your side of the contract despite them not paying you.
Above all though, for that much money owed for that long, I'd be looking to recover whatever I can as soon as possible in case the company declares bankruptcy. There are many strategies that might be used, but I'm not a lawyer and my knowledge of the relevant law outside England and Wales is slight. Though if they do go bankrupt the reciever might actually be easier to deal with than the company directors, and may be more inclined to pay you something, even if not the full amount due, to keep the company in business as a going concern.
If the company wants future services I'd also be inclined to start requiring at least part-payment in advance.
So I'd say yes, you need a lawyer. Unfortunately.