Because of how the WCA is worded, it is an offence to set a trap (not listed on the stoat general licence), where it is calculated (England & Wales) or likely (Scotland) to cause injury to an animal in schedule 6 or 6ZA.
As it can be difficult to avoid stoats in some instances, given their size and agility, where they are known to be present an approved stoat trap which is also authorised for other species should be used.
In other areas where stoats are not known to be present and as such catching one is not calculated or likely, you should be able to continue trapping other species such as rats with appropriate traps.
If a stoat were to be caught in an uncertified trap (i.e. one not listed on the STAO for stoats) without it being calculated or likely, then it is unlikely that an offence would be committed.
But ultimately, it would be for the individual setting a trap to assess a given situation and potentially be prepared to justify their actions if they were to be prosecuted.
It is also important to note that it is not only if a stoat were to be caught but in situations where it is calculated or likely to cause injury that an offence would be committed.
We are seeking further clarification on this and hope to update in due course.