Depending on how the indicators are wired may need two buzzers. As there are often diodes in the circuit.
I fitted an Audicator 3 years ago and find it excellent. It ramps up the volume so you hear it even when the car is very noisy. You can set the time it starts bleeping and it is silenced if you touch the brakes.
Thanks to everyone for advice, guidance and suggestions.
Doug at Just Add Lightness has very kindly sourced a relay with buzzer from one of his suppliers.
Great service from JAL
Audicator - 15% off for L7C members.
Audicator | Lotus Seven Club (lotus7.club)