You may remember this: http://devos4.blogspot.co.uk/2016/11/searching-for-head-or-graphic-novel.html
Well, chasing up another lead, I came across these: https://mastercrafted.co.uk/shop/infantry-components/heads/hellmask-heads/#reviews
You can see that one review clearly says "They arrived quickly, were of very high quality and helped make my Helghast army absolutely perfect."
So that's that then. They do exist. However, I have tightened up the rules of acquisition this year. And have already blown £250 on half a project (more on this at some point in the future). So further things (except the rest of the kit to finish the SAM Project) will have to be birthday and Christmas presents.