Gary -in the installation manual I'm looking at, on p26, 5.3. Functions Settings, under SW2-6. (mixing tank) it states that when SW2-6 is off 2 Zone temp control is not available!  And under SW 4-1 multiple units (which I don't have) it states that when SW4-1 is off 2 zone temp control is not available.  I'm getting very confused!

Derek - thanks for that.  Zone 1 was set to Room RC1 (I assume that's because I have the wireless thermostat ).  I've changed it to main RC to see if it makes any difference but I can't leave Zone 2 unselected.  It has to be  TH1, MainRC or Time/Zone.  I've tried all of them.

@bryanb There are known to be mistakes in the manual, so don't treat it as gospel, afaik SW2-6 is if you have a mixing valve to have different temperatures in the 2 zones so hotter rads and cooler UFH, if you dont have that it should be set to off.  SW4-1 is definitely if you have more than one outdoor unit this is nothing to do with 2 zone control so again I think is a mistake in the manual and both SW4-1 and SW4-2 should be set to off.

SW1-8 should be set to ON if you have a wireless controller, presumably that is the case and by wireless controller I mean this.

If the above controller is on zone 1 that should be set to RC1 and if zone 2 is using a 3rd party thermostat should be set to main RC.

This is the main RC.

I'm not sure any of these changes will impact the error you are seeing but its worth a shot.

Thanks for your time with this Gary.  I have a wireless controller running Zone 1. Sounds like the manual is more of a hindrance than a help!  Your comment re SW4 1& 2 make sense.

I think though if I didn’t have SW2-6 on it gave me an LL error which was to do with incorrect DIP switches.  I’ll check all that again when I clear out the airing cupboard to get at the control box.

Had another go at this, the morning.  With SW2-7 ON it gives me a flashing LL error at the top of the main controller, but it still appears to run normally. However it does not seem to set up a Zone 2.  The bottom left button on the controller is still marked "I" and I cannot set up Zone 2 room sensors.  If I turn on SW2-6 I don't get the LL error and can set up Zone 2 settings - left button on controller marked as "Z1 or Z2".  But then after several minutes the L5 error comes up and everything shuts down.

I guess if I turn on SW2-6, it's looking for a mixing tank(which I don't have) but presumably would have a thermistor which it can't find, hence the L5 error.  I have what I think is called a low loss header, but that's hardly a mixing tank, with no temp sensors.

Not sure where to go from here!