Thanks for the comments guys. Just to be clear I am getting a signal from both the Wahoo and Coospo, it’s just that it doesn’t seem to follow my perceived heartrate.
After a while the straps seem to get clogged with salt or the sensor pads delaminate (not really sure which) and this also gives dodgy readings.
The Wahoo strap looks clean so I am now trying a new battery with that, although it doesn’t seem along ago since I replaced it. The Coospo strap sensors do look quite manky, so it’s in the washing machine today! I’ll see how they work on Saturday’s ride before deciding whether to go for:
A replacement strap without electronics is only about £10 on Amazon.