Personally, I think you need a new mono-bloc but I don't think you need a new dish as they don't really deteriorate until there is heavy corrosion (sea air or similar) or they have been damaged by being hit with something (or swung past the dish E-W limits and hit a wall or something like that).
Changing a mono-bloc is a LOT quicker, easier and cheaper than changing a dish, and you would still need a new LNB anyway even if the dish did prove in the end to be faulty - so change the LNB first and then see whether that has fixed the problem before going any further.
PS: just looked again at the pics of your dish and it looks very like a very old 80cm perforated AMSTRAD SKY dish - and in pretty good overall condition (can't see any physical damage or corrosion). Just binned one of those but only because of the slightly bigger s/h dishes that I had collected over the years and am now setting up.
Only problem I can see is that you may need to be careful in loosening the bolts holding the LNB to the arm because the original AMSTRAD plastic LNB holders get brittle after a long time - but I seem to see that this has already been replaced by something else. Anyway, spray some good quality penetrating fluid on all the bolts around the LNB a few days before you actually want to change it - and the bolts and screws should then come apart without too much problem.
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