reports varchar link

I have added a custom field to a custom module so it can be shown in reports.

The field contains a link to a report (with query string values added with the intention of setting the filters with those values)

The field currently shows on the report (rows-columns layout) as "varchar" which seem to use the BASE field LIST.hbs

I can't work out whether I can create a custom controller and layout to make the value show as a link.

Is this possible?

I tried adding a custom 'base' controller and layout but this seems to be ignored (perhaps my extendsFrom value is wrong?)

I tried adding a custom 'varchar' controller and layout but this produces errors and leaves the values blank on the page.