Reawakening the thread once more having carried out a successful hack as per 960kg instructions. Left and right channel has individual 2.2uF 250V caps soldered in and the ground cable is clamped down with the screw next to the antenna socket.
These are the caps I used:
https://www.switchelectronics.co.uk/2-2uf-250v-metallised-polyester-film-capacitor-10
I believe the caps are there to filter and protect the device at the other end?
Anyway, it works perfectly! I've plugged my 3.5mm jack into a bluetooth audio dongle thing so I can pair any phone with it to play tunes or whatever. I used a dummy cassette with the tape removed and reels left in.
Massive thank you for making my 4 hour trip to Cornwall tomorrow more pleasant!!
Sam