It seems a few things still need adjusting...

For the record:
All processing done in Microphen 1+3 at 30 deg C - not much I could do
about the temperature as it was hot and I didn't bring ice cubes...

I processed the first sheet (shot with filter) for 3 minutes,
equivalent to 10 minutes at 20 degrees.
There was hardly anything to see - only the highlights showed at all.

Then I did the third sheet, which was also shot with filter, for 6
minutes (20min@20degC), which gave a very light negative - I'd say too
light as the shadows are practically transparent with hardly any detail.

So then I tried the second sheet, the one overexposed without filter,
and used 2.5 minutes (8.5min@20degC) for that, which resulted in a
negative that's on the light side but shows good detail and looks
rather printable, if a bit soft.

Overall it seems that 1 ISO might be a bit optimistic and indeed I now
recall that the guy on flickr I referred to earlier said he used 0.75
ISO. (Darn, that's -6 DIN!) And that's while push-processing his film
by a stop. So I guess next thing I'll try something like 0.75 ISO and,
say, 20 minutes (at 20 deg C) in Microphen 1+1. For starters, then
take it from there.
Any other suggestions, anyone?

Cheers,

Ronald