Hi Head78, from memory, when I got my exam results (the very next day to my amazement!) it didn't state any validity on there per-se. Although I believe that IMMI requirements are that you have to have taken the test in the 3 years immediately prior to lodging your application.
The other nice thing about PTE was that they only book 8 - 12 people on any one exam (certainly in Cardiff where I went anyway), whereas IELTS was 100 candidates at each exam - It was like being back at school taking GCSEs. I was actually lucky on my exam day though as I was the only candidate booked to take the exam, so I had the room all to myself. It goes without saying also that PTE being completely Computer based is far better in my opinion than traditional exam papers and question sheets like IELTS is based on.
I definitely recommend you take the practice PTE exams though online and read up about the exam format, as it helps to know how the Computer exam will flow on the day, since when it starts you cannot stop it and you have to keep going with in the time limits for each part. Sounds worse than it is, but it's straightforward once you've read and understand the format.