For a few years I fell on some hard times and had problems repaying debts. I was out of work. I currently have no poursuites and never filed for bankruptcy and am back in a well-paying job.

However, my credit score suffered massively as a result and I cannot now get a leasing for a car. I regret having applied for a leasing as I understand this has further impacted my score but what's done is done.

How does one go about increasing their credit score with ZEK?

First, that is good, that you are back on the rails.

There is this page for simplified general information on credit checks in CH (it exists in English as well, just pick EN from the languages list).

https://en.comparis.ch/privatkredit/bonitaetspruefung

To learn about the period, while your misfortunes will affect your credit checks with ZEK, you can address this document:

https://www.zek.ch/getattachment/ec9ffde2-9fc0-440f-b38d-34bdbde0fc5a/Liste-Aufbewahrungsfristen.aspx

from this page:

https://www.zek.ch/de-ch/kredit-leas...ahrungsfristen

What can improve results of your ZEK checks is a well maintained loan from any ZEK partner, willing to give you one. Depending on your code you may or may not have a chance to get a loan. Following are some information about this:

https://www.credix.ch/kredit-blog/29...K-was-tun.html

Hope, that it helps.

Many thanks for all this, will take a look.