Many of the 0.2W resistors are too long for the slots provided and really need to be mounted vertically. I think you should mention this.

C77 as supplied was too fat to fit. The components around here are very crowded and I had to mount the resistor above C77 a few mm off the board to make room. What with this and C77 and the capacitor underneath C77, it was very difficult to get the MC1310 chip into place. You need to rethink this, or at least rethink what you supply for C77. I would have thought a little ceramic disc would do at this point.

I didn't find the documentation about the relays and the 14-pin header very clear, although I got it in the end. You might look at this again.

I measured >= 38dB separation in both directions at 1kHz, which is very good, not quite as good as the chip specs but still very good. Reduces to 15dB at 15kHz.  This was straight out the signal generator, not via a tuner - see below. I then measured the distortion at about 1.3% in mono and 2.3% in stereo, in both cases *including the tuner* (Troughline III, whose own THD is about 1.2%). It looks very clean on the scope too.

It is pointless adjusting the separation control P2 using the composite output of the FM generator straight into the decoder. It must be adjusted with the composite signal going right through the tuner in question via IF or RF injection, i.e. just like a broadcast. Otherwise you're just assuming that the tuner's audio response 23-53kHz matches the generator's exactly, which it won't, especially for valve tuners with IF transformers, which generally roll off pretty savagely in this region even when correctly aligned. The separation control is where you match the tuner to the decoder and it is an essential step. After doing this step again via the Troughline I couldn't get better than 30dB separation, at a considerably different setting of P2 from above, but that's what the tuner can do to the L-R information. Probably the Troughline's alignment can be tweaked a little further but nothing dramatic.

In fact I would have liked to see a facility on the input side of the decoder for adjusting the incoming frequency response - just a parallel RC pre-emphasis is required, where the R can be a trimpot.