I have a driving licence which permits me to ride a motorcycle up to 125 cc, does the same apply in Spain. What are the test requirements there and what can you ride without the need for a test?

Your UK driving license is valid in Spain. You might find these links useful >https://www.confused.com/on-the-road...%20in%20Spain.
https://www.citizensadvice.org.es/af...ence-in-spain/

Enjoy

Bob's Tuono @ Sanctuary

But after Dec 31st. only if you are a visitor and not resident.
If you are resident, or in the process of becoming, then you will have until the end of June 2021 to exchange it for a Spanish one or face taking a Spanish test.

I don't think that correct. From everything I've read and from what our solicitor has said the application to change your licence must be in the system before 31st December. And if course you must also already be a resident.
I'm off to start our residency application tomorrow and I will get the official word then.