Bertie -
A word of warning here, based upon your statement that you are a bit close to the $50,000 income level required for the visa.
Assuming that you get the visa, then when you come to renew it in four years' time you will again have to prove that you have an income of a minimum of $50,000 per annum.
At that stage you will 'only' be investing $250,000 in your Designated Investment - so your income from that will reduce. You may need to find a suitable Term Deposit for the other $250,000 in order to maintain the same level of income in Australian dollars.
And although we desperately hope that the exchange rate from GBP to AUD will not go much lower than it is now, do make sure you have some safety margin in your calculations.
As a benchmark, the UK occupational pension income I declared back in 2006 for my initial 405 visa would have been worth only about 70% of that amount when I sent in my renewal in June this year, if it hadn't been offset to some extent by my pension being index-linked.
(And the exchange rate has experienced another 8% fall since June!)
I can't imagine another fall of that extent in the next four years, but who would have predicted the falls that we have actually experienced recently?
Regards,
Howard