Just to update with what happened in case it helps others:

I rang the UK Customer Service number and it went through to Dubai. The guy I spoke to was extremely helpful and confirmed this is the correct procedure when inbound dates are off the system.

Make a booking using the correct outbound date and the last available inbound date (was 19th January 2018 on his system; 8th January 2018 when I go online so I presume EK has a bigger window).

He gave me the booking ref and his name and once the inbound date is up on the system I phone up quoting both, give our actual date of return and it will be changed for no charge. The ticket will be updated after that.

The guy spoke to his boss and got approval to not charge the normal phone booking fee (£10 each) saying that would be unfair as I couldn't book online to avoid it.

Before finalising the booking and me paying he confirmed all the conditions - cancellation and 2nd change fees, blackout dates on Saver fare etc.

I can select outbound seats, make chaffeur bookings etc on outbound flight but of course will have to wait until the inbound date is available and confirmed before doing same with it.

Job done!