Sci-fi show where people were turned into creatures that looked like mops (seriously)

Hi all; I'm looking for help identifying what I think is a 1970s or 1980s British science fiction TV series. I don't remember much about it but it was in colour and seemed to be set in some sort of caves, and there were these weird kind of creatures who looked to me like (don't laugh) they were made of the strings or yarns on a string mop. It was as if you got a giant mop (minus the handle) and placed it on top of someone so that they were completely hidden and all you saw was a giant mop head slowly coming for you. I was very small, and I'm guessing I saw it on TV in 1982 or 1983. Anyway... In at least two episodes, the process of converting people from regular people to these creatures was depicted, and this was achieved by the camera looking elsewhere, then back at the person at each stage of the process, and then away at something or someone else, etc. I seem to remember four stages in this transformation. Firstly, it looked as if the person was covered in a mass of cobwebs. (Remember, you don't see these cobwebs being applied; the camera looks at the person and it's already happened.) Second, I think it looked like the person was covered in some sort of red or brown gunk. Now, I can't remember the third stage, but the fourth and final stage was that person now resembled this mop-type creature I described.

Thinking back on it as I write, maybe I'm describing stages 1 and 2 in the wrong order, I don't know. And who knows, maybe there were just three stages. But either way, people were transformed into these "creatures" with no faces, arms, legs or any distinguishing features, except that they looked like they were made of the material that comprises a string mop, or at least to a three-year-old. Thanks.