Hi Dave and Medium RoastSteam, thanks for your reply.
I think the shape of the O ring is indeed the reason why other gaskets do not seem to fit. When newly installed, it will seal without me having to lock it in very firmly. I use the stock PF with the blind disc for backflushing and although that one can be inserted as far as the 5’o clock position, with either PF, the gasket will seal well with a relatively loose fit. After some time (maybe 1.5-2 months) of use, however, it will start dripping a bit of water from the rim, so I do need to lock it in more tightly. At this stage, I inspect the gasket and if still usable, I put it back in with a sheen of Molycot along the outer surface. Once it starts leaking again, I often find the rubber has become brittle and when I bend the O ring, it snaps. This typically happens within 3 months after I have installed it new.
I must say that I regularly do leave the machine on for most of the day (say from 7.00 till 22.00) when I am working from home and during weekends. I typically make espressos in the morning and one after supper and use the hot water tap for tea in between.
I will see if restricting the use of the BZ10 to morning hours only makes a difference; I can use a kettle with Brita-filtered water for tea and use my Robot for my evening espresso.