How to continue learning
 

We moved to Spain with my then 2 year old 18 months ago. She has been in nursery full time since then and has some understanding of Spanish. We are having to return to the uk but would like her to continue learning spanish so that skill isn’t wasted. Obviously we can’t replace 6 hours per day immersion but would like some suggestions how best to continue developing her Spanish.

Thanks

There are many books written for children in Spanish. If you read them to her, and help her to sound out what's written on the pages, that would be very helpful.
If you don't speak Spanish yourself, I suggest you find someone that does.
Perhaps there's a Spanish speaker that works at a nursery.

Some children's books are written in both languages. I think those would also be good. Your daughter can then hear and learn both languages at the same time. (And those stories may also help an English-speaking reader to learn a thing or two a la vez ;) .)

I'm a school fan, so adding to Rusty's great advice, if you can do it, you may want to enroll her in a formal course. At least two systematic hours of study per week will keep her learning active. :)