All the numbers I have looked at say that there is sufficient space/geography available in the UK and Australia to install pumped hydro storage capacity to enable 100% renewable generation for each nation respectively.
Really? Where in the UK? (I make no comment about Australia)
What does your unnamed source know that David MacKay doesn't? See http://www.withouthotair.com which is lauded by everybody with any axe to grind.
You can point to that reference all you like, it devotes substantial length to the matter and supports the realistic view that the UK has sufficient space and geography to store enough energy to cope with lulls in a largely or 100% renewable generation scenario. It trivially finds enough storage capacity in a small number of existing unutilised water bodies to store 2 hours of operational time in addition to the current estimated 30 minutes of hold up time. This is without considering the capability to build new storage capacity, increase the capacity of existing locations, tidal lagoons, etc.
Nonsense. In each of his 5 plans for the future (see p203 onwards) he keeps pumped hydro at the same level as today - for the good reason that most of the good spaces in the UK are already fully utilised.
As for the capacity, the combined capacity is 30GWh, sufficient for significantly less than 1 hours UK electricity consumption. Given the intermittancy of wind and water power in the UK, that is completely and utterly inadequate.
As for referring to Mackay as a good source - of course I do. Everybody, from Greenpeace to Big Energy, regards his work as an excellent starting point for discussions.
If someone makes claims that are extraordinary compared to that starting point, then that someone has to provide extraordinary proof. So far nobody has been able to provide sufficient proof that stands up to examination. If you can provide extraordinary proof for your extraordinary claims, then everybody will listen.