Client not paying, ignoring phone calls, what to do?

Hello, I'm new around here so I hope this is an appropriate topic to post.

I did some design work for a client way back in November 2016, I invoiced them at the end of the month for the total of the work which we had agreed prior via email. I have various emails and texts from the client saying the cost issued would be no problem. I've been chasing them ever since and have yet to receive any kind of payment. The total cost is £1400.

April this year they told me via text that they have had a bad Winter financially but have not forgotten about the invoice.

After more chasing they have now started to ignore my calls so I'm at a bit of a loose end as to what to do.

I've recently sent them an email saying I will be taken action further if payment is not received soon. And I have been looking into sending letters as a more serious take on the matter.

What are my options here? I've been looking into maybe getting a company like http://www.safe-collections.com involved but I don't want to have to pay out more money in order to get this issue resolved. If the debt collection company can retrieve the amount due + pay for their services then great - is this usually how it works?

I've also read that I can add some kind of late payment costs to the bill - is this correct and if so, how is this calculated?

Any help would be appreciated!
Matt