3/8" suggests BSP, i.e. the size of the float valve's threaded inlet.
Does the loo come with a tap connector/isolation valve? If so, this is probably 3/8" BSP female at the end which attaches to the float valve, and accepts a 10mm pipe. By using the flexi you mention, you would disgard the above and use the flexi in its place, taking you straight from 15mm copper pipe to the float valve's inlet.
The toilet comes with a fmale outlet and then they supply a nut, rubber washer and split olive. The instructions say to connect a 10mm pipe.
Given that it is German I wasn't sure if the fitting was 3/8ths or a real metric. Others faced with the set up have suggested 3/8ths to 15mm flexi hose- I shall pick one up and see how it goes.
Opps just re-read your post- i am going to connect the 3/8ths to 15mm flexi to the underside of the cistern at the male connection (and leave the internal flexi in the cistern as is).
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