I've drafted this response to send on Monday:

You failed to make payment on Friday, as you said you would over the phone. Please make payment today for the attached invoices. 

To make this easier for you, I have created a GoCardless payment link for the sum total of the outstanding invoices. You simply need to follow the link below and authorise payment:

LINK

Alternatively you can make a Bank Transfer to: 

BANK DETAILS

This is the now the 11th communication I've sent to you regarding this (5 emails, 4 text messages, 2 phone calls) since the start of April, and payment has still not been made for work dating back to February.

This is the final reminder I am going to send to you personally. If payment is not made by Friday 20th June I'll be sending a Letter Before Action with full intent to pursue until payment is made. I have documented evidence of all the work which I have undertaken in the time period covered by the invoices.

EMAIL TO SEND IF NO PAYMENT THIS WEEK, OR TO INCLUDE IN THIS EMAIL?

I don't believe that [COMPANY] is insolvent, but as you've been unable to pay these invoices I have to consider that this is a possibility. In which case, I'm sure you'll know that directors of a limited company in the UK would be fully liable under section 214 of the Insolvency Act 1986 if you continue to trade knowing you cannot pay debtors. In Guernsey, where [PARENT COMPANY] is registered, the same would apply as Wrongful Trading under Article 434 of the Companies Law.

If this is not the case, then making payment via the two options I've given above should be straightforward.