I am enjoying my PhD but its definately not a field i want to dedicate my x-many years of research to POST-PhD. Is it difficult to change fields (in my case, from Cardiovascular to Neurology) and would it mean i would have to undertake a research assistant/technician post before i could go onto a post-doc position?

YES!!!! To put it simply. I am sure it depends on the field, where you live, what your PhD was in etc. But, I am in this situation right now, and it is proving downright impossible for me to get a job just one tiny step removed from my PhD topic, even though the jobs I am applying for require the same skills that my PhD has given me. And I am not even dreaming of applying for postdoc positions, instead I am going for research assistant/research associate jobs at unis which do not even require a PhD.