mike writes:

[...]
> neighbour some little time before this started.
>
> How could I detect external interference?


So neighbours are bad for DCF77!? ;-) Maybe move your antenna closer to your
neighbour, then further away, and compare the results: Most interferences are
short range, unless he has a high-power device.

If you can build a 77kHz receiver and make the signal audible (it's AM with FM
also), you'd clearly hear interferences (Normally you should hear a clear tone
with amplitude varying once per second (AM) with a little "scratchy" noise
(FM). If the sound is more like frying a steak in a pan, you have
interferences ;-)

(I'd wish commercial receivers had an audio output for positioning the antenna
perfectly)

Regards,
Ulrich