I found kolab which appeared to do all we wanted and used KDE PIM so I embarked upon setting up a new server to try it.
On looking for the right download it appears kolab is no longer supported. The forum has been closed so no community forum and download versions offered all relate to older and now dated versions of operating system.
Since we wish to use this in anger if it works I have sent sent three messages to the commercial service kolabnow and have had no replies in a week so they clearly do not want our business. I wonder if kolab is dying or whether I am just looking in the wrong places. Please could somebody advise.

It's been a while since I've heard much from that realm or from Aaron Seigo since he joined them a few years ago (a little sad as he used to be one of the biggest driving forces behind KDE, Plasma and Free Software).
At least it looks like Aaron may have switched to RoundCube which Kolab had been collaborating with and which is still in active development.

Hi airdrik,
Thanks for the update.

I persevered with this and after a bit of snooping found the latest version of kolab, Windfall, a beta version of what I gather will be kolab 18 on the kolabsys repo site. They even have a version for debian 9 so all is not lost.

Rather than spend a long time trying to set up my own system, a task well beyond my capabilities, I signed up for a single user monthly subscription as the quickest way of finding out if their product worked for us.

Now my reason for looking at kolab, as stated in my first post was that it is reported as being based on the KDE PIM and what I need is for the calendar app to work in the same way as KOrganizer (and T'bird/lightning btw), does on the desktop.

Specifically; that if you right click on a day or period in the calendar you are offered the choice of creating an event or a task. If "task" is selected you can set up, for example, a repeating task, as might be used in a maintenance calendar, with resourcing etc. and once saved, the task appears in the calendar for every instance. You do not have to go to a task window and work with a list, although that is created too.

Sadly I have discovered that the webmail calendar which is available from my kolabnow account does not behave in the same way so I cannot create and see tasks in the same calendar window as events.

I have asked for technical support which my subscription makes possible and am hoping they can address this point but I am not holding my breath. Will report back if I get problem solved because if I do kolab system will offer exactly what we need for managing our small business.

Thanks again for your reply.