1 You came to the UK permanently (HMRC 6 part 7 (7.2))
2.You were a long term visitor, ie you intend to stay in the UK for at least two years (7.4)
If you were a short term visitor (intends to stay less than 2 years) the 90 day rule is relevant, I assume this is not the case.
So assuming 1 or 2 applies you are treated as UK R in 09/10, again assuming 1 or 2 applies you will qualify for split year treatment.
Your residence in 09/10 will therefore be from Sept 09 to the end of the tax year (05/04/10)
Yes, 2. applies since my intention was to stay for more than two years when I moved there. I'm surprised you say that the residence started in September 2009. I thought you were considered a resident for the complete tax year, even though split year treatment applied.
To establish whether you were TNR the DTR must be considered and I believe would be judged from a UK perspective of the UK tax year and from an Austrian perspective to be the calendar year.
From an Austrian perspective I think it's pretty clear that I was a resident in 2009 since I lived there from January to August and had no home elsewhere and kept a home for the whole year. From a UK perspective, I'm not sure and I guess it depends whether I have to take April 2009 to April 2010, or as you suggest above, September 2009 to April 2010. If the former, I'd say Austria clearly wins. If the latter, it's less clear.
I guess my confusion stems from the fact that CG26540 says "there was no time in that year when the individual was treaty non-resident". What does "no time" mean? It suggests to me that the year can be split into different parts but I find no information or examples on the HRMC web site.