These are the 868 MHz modules from Conrad – receiver on the left, transmitter on the right:
Worked as straight replacements in the 433 MHz setup, but there seems to be some problem with noise. It got a bit better when the receiver was tied to an 82 mm wire as antenna (as shown), i.e. exactly 1/4 wavelength. And that funny transmit antenna wire/coil probably isn’t optimal either…
Still, a bit noisy, lots of spurious pulses, and only occasional CRC-valid reception. It looks like much of the problem is caused by the crude bit-start synchronization software. Needs more work, clearly.