I want to send a paper to be reviewed in an academic journal. An early version of this paper is available on a website as part of a grant I received. The early version is not in a journal nor is it reviewed. But one can read it online. The grant programme encourages publication of findings in peer-reviewed journals. However I am not certain whether or not this is considered 'previously published' material? Any advice?
That's probably OK (hard to be sure without specifics). In several areas, it is quite common to post preprints (e.g. in the ArXiv https://arxiv.org/ ) and those do not prevent one from submitting it to a journal. Some areas (e.g. CS) are even OK with publishing preliminary versions in conference proceedings. What is not allowed is to submit a paper previously published in another journal or even submit a paper to more than one journal simultaneously. I think everyone frowns on that.