"Any ideas for the best 12 month validity ticket options in Y (flexibility not vital, but helpful even if change fee involved) ex-LHR - or are there better options from other European ports? Departure/return ex-UK in northern hemisphere summer."

There are lots of choices. Many tickets to Oz have a 1 year validity and nearly all allow date changes. I got a LHR-SYD in June for £560 including tax with Qantas. I would suggest just going to somewhere like www.airnet.co.uk . Give it a date and route and ask for "full fare list" and it'll return a list of fares which are valid for that date that they offer and also you can link in to see what the max stay,routing,stopover and change rules are.

Dave