Ok, just had a response from UA - apart from the usual guff reminding to do things I already did, there may be some progress. Here's what they just sent me (anything in bold is my emphasis). Thoughts/comments welcome as ever:

"This is to confirm that we received your concern about your Mileage credit along with the copy of your supporting documents.I've checked your account and it appears that the TG 917 LHR-BKK, TG475 BKK-SYD and TG916 BKK-LHR flights have already been credited. To view your account activity, sign in at www.united.com , go to the “My Account” page and select “Recent activity” from the “Balances & status” header.

To ensure you receive proper credit on future flights:

- Make sure the name on your reservation matches the name on your MileagePlus account.
- Provide your account number when making reservations and at the time of check in at the airport.
- Keep the original boarding passes and a copy of your ticket receipt until you see the flights credited online.

Moreover, for your TG462 MEL-BKK. We’re working with our Star Alliance partner on your behalf to receive authorization to credit your missing flight activity. Please allow up to four weeks for resolution.

To see if the flight(s) have credited, sign into your account at www.united.com/myaccount and from the My Account page, click on Recent activity from the Balances & status header.

If your mileage hasn’t posted after 4 weeks, the flight may not be eligible for credit. Kindly message us back or call our MileagePlus Service Center at 800-421-4655 for further assistance.

Please note: Our partners will not accept requests for flights more than 12 months after the travel date, regardless of the original date of submission."