I have answered my own question! I found the hack below and it worked on my Sony BPD-A6000 player.
You will need one of those One4All universal remotes and it needs to be one that is programable, I used One4All URC 7960 remote, but I'm sure others will work OK.
1. Press DVD
2. Hold the magic button until the DVD button flashes twice.
3. Press 0533
4. Hold the magic button until it flashes twice.
5. Press 994
6. Press Magic Key (don't hold)
7. Press 00189
8. Press 1 (this sets it to key 1 - it flashes twice)
9. Hold the magic button until it flashes twice.
10. Press 994
11. Press Magic Key (don't hold)
12. Press 00255
13. Press 2 (this sets it to key 2 - it flashes twice)
14. Hold the magic button until it flashes twice.
15. Press 994
16. Press Magic Key (don't hold)
17. Press 00095
18. Press 3 (this sets it to key 3 - it flashes twice)
19. Hold the magic button until it flashes twice.
20. Press 994
21. Press Magic Key (don't hold)
22. Press 00221
23. Press 4 (this sets it to key 4 - it flashes twice)
24. Hold the magic button until it flashes twice.
25. Press 994
26. Press Magic Key (don't hold)
27. Press 00079
28. Press 5 (this sets it to key 5 - it flashes twice)
The remote is now programmed and ready to use, to make the BPD-A6000 region free.
29. Use the remote that came with the player to bring the player out of standby and make sure there isn't a disc in it.
30. Press 1, 2, 3, 4, 5 on the oneforall remote. The message 'OK' will appear on the DVD display.
31. Put the player back into standby with the Sony remote.
32. Next time you turn on the player it will be region free for DVD playback.
