Thanks.
3m is exactly what I need. I was saying the 12m is too long, which could cause issues with interference.
Note that it's wired as a crossover cable so you'll need a joiner that accounts for thatI see now that it says the pins are reversed...
Is this one ok? https://www.amazon.co.uk/kenable-RJ12-6P6C-Cable-BLACK/dp/B0051FKFXQ/
If not do you think you could point me in the right direction? (US sites are fine)
Thanks again!