I don't know anything about Seville in Spain, but we've priced up a weekend there mid-January really cheap. At 6-15 degrees C during the day it'll be much warmer than the UK. We have a (will be) 5 month old DD, is it suitable? Any travel tips for Europe with a small baby?
Seville is beautiful. Bear in mind that most toilets are tiny and have no baby changing facilities. It is very family orientated and you will see the locals out with the very young kids until midnight or later. There are some fab shops for kids clothes.
Make sure you have a folding changing matt in your bag We have a place in Spain so have changed babies there frequently and it was unusual to find facilities- it was usually a (tiled) floor or laid flat in the buggy