Need advice & solicitors letter in chasing non-paying customer

I am a web developer, and have a situation where a client, despite continual promises to 'get it sorted right away', has just not paid me.

Work was completed in early April after a deposit was paid, and the customer has been enjoying the benefits of 1000s of visits to the pages created since this point. Payment terms for the balance are on receipt, and the customer has never contested payment - simply never makes the payment he says he is going to make.

We're now more than 6 weeks down the line, which might not seem like much, but as a small business having a week's worth of revenue missing causes serious issues.

I've sent numerous emails requesting payment, and have been told over the phone on at least 4 occasions that the payment will be made that day.

Really starting to lose the will to live here, and have consider flicking the 'killswitch' on the site he has to remove it from service - but am aware there are potential legal issues with this.

Going forwards, I would hope that the successful bidder here may be able to help with getting a number of contracts in place to help avoid this kind of situation in the future, and so please advise whether you can help with this going forwards too