Article 17 of the UK/Australia tax treaty here
deals with pension income. As a UK resident, any pension income (as opposed to a lump sum) you receive will be taxable in the UK.
ESC A10 here
gave relief from UK for lump sums paid out from foreign pension funds depending on the length of service and the proportion of that service carried on outside the UK.
ESC A10 was enacted into legislation with effect from 6 April 2011 as you will see here
. As far as I can see, given that you pension fund appears to relate 100% to working outside the UK, any lump sum you take should be UK tax free. Although ESC A10 no longer exists, it effectiveness applies to pension rights accrued before 6 April 2011 so you would appear to be covered by both the old and new rules.
If I were you, I would ask HMRC for one of their technical sections that specialises in foreign pensions to look at your situation. Your letters may have been passed to such a section and it can take some time to get a response. It's worth chasing them again.
I hope this helps but let me know if you have any further questions.