Hi Jemma,
Welcome and congratulations on your new post. Hope it is going well and you are enjoying it.
With regards to SEND baselines; what baselines are you using as a whole school? Lots of schools use something like GL assessments and this will be used for all pupils to find their baseline for learning and progress to be tracked from. Any whole school testing can be used to support the identification of SEND pupils and you can then look further into the areas of difficulty. Once you have established the areas of difficulty and where the support is required, you can implement targeted, impact driven interventions. High quality intervention packages will also then include a baseline test pack within them that allow progress to be monitored throughout the intervention program.
There are baselines for SEND pupils with very complex needs, however they are normally used in specialist provisions.
Hope this helps,
Kate