@yellowymellowy Happy to help!
Is it the case that, because of the QE in 2009/10, the already very low interest rates and high level of government borrowing when we entered this, the situation is even more precarious as there are less interventions available to them?
Yes, exactly. We didn't experience a "real" depression in 2009/2010 because of significant amounts of QE, interest rates hitting all time lows and government support schemes like Help to Buy etc. Unfortunately now, we're running out of effective interventions and what we're doing isn't going to contain or mitigate the coming recession.
How much can policy like permanently suspending the stamp duty (I suspect Rishi may extend this) do to maintain prices?
Very little and it's a measure that will fizzle out come October. The stamp duty cut was rolled out to prop up falling prices and bring some confidence back to the market. Unfortunately, a saving of 3% on a house isn't good enough when there's a recession of this magnitude.
How much is Brexit going to affect all of this
It really depends on our trading relationship with the EU. 51% of all imports are from the EU and tariffs (WTO terms) on these will mean increase in prices. If you take food as an example, the UK imports 30% of all food consumed from the EU, the British Retail Consortium said that the average tariff on food imported from the EU would be over 20 per cent. That would mean a significant increase in food prices, resulting in a significant increase in inflation.
...and is it likely that all this QE will start to lead to too much inflation and potentially a rise in interest rates?
The reason I separated this out of the Brexit question above is because this is a highly complex subject. QE can and will lead to inflation if demand remains stable, however in this instance, demand has dropped so QE won't affect it by much until demand comes back. Now, if you see my response regarding Brexit, you will see that there is a chance inflation will significantly increase depending on our trading relationship with the EU and that's because supply side pricing would have shot up. If that's the case, then in order to counteract inflation, interest rates will have to go up.
Hope that makes sense. Feel free to ask if not.