Thank you for your help and advice - I wasn't aware of this - something else to add to a growing list of considerations when defining names...
Initially, the 6 bulbs were named Irish 1, 2, 3, 4, 5, 6
The GROUP worked fine but control of individual bulbs in the GROUP was limited to On/Off
Someone suggested changing the format to Irish_1, 2, 3, 4, 5, 6
I tried this but it didn't make any difference.
I have also tried:
Deleting these 6 bulbs and the Group in the Alexa App
Deleting these 6 bulbs in the Hue App, adding them as new bulbs and renaming them
Re-discovering the 6 new bulbs in the Alexa App
At this stage:
All 6 bulbs work as they should (on/off, colours change, brilliance %)
Next:
Create a new GROUP in Alexa App
Choose a different name for this new GROUP
Add any 1 (or more) of the 6 bulbs to this GROUP
At this stage:
I can control the GROUP (on/off, colours change, brilliance %) - all bulbs in that GROUP respond simultaneously
However, control of individual bulbs in that GROUP is now limited to on/off (I can not change the colour or brilliance %)