My thanks once more to the invaluable help offered here.

Frankvb and Crampedin13A you have alerted me to crucial information which would have detracted somewhat from the trip.

I have now been able to change the date of flights to 19th June returning 6th July in First out and Club back. Again no charges for the change because the "web site would not allow me to make the change myself" although the taxes and charges increased again by 50p per ticket. (I have no idea why that should be but there we are).