I'm curious as to what you mean by 'it's likely you'll be working four days a week in term time'? Do you mean that it's likely I'd need to be on campus four days a week, or that there's likely to be extra work beyond the 2.5 days required for the role?
I think the fact that you don't understand the way that university teaching terms/vacations work, and the way the workload differs between term time & vacations, means that you really do need to do a bit more research about this post.
As others have suggested, if there's an "Informal enquiries" phone number or email, do use it, to ask about the spread of work.
Basically, in my experience (over 3o years & several institutions), am 0.5fte post is rarely 2.5 days a week throughout the year.
But before I can answer your question, I'd need to know whether the post you're thinking of is a teaching & research post, or just teaching only? (it's often called "teaching & scholarship," rather than "teaching & research").
If it's a Teaching Fellow or Teaching only post, then you will be expected to take a proportionately heavier teaching load than your teaching & research colleagues, who have a third of their time allocated to research - I'm about to go into a regime of 1,000 words a day to get a book finished by the end pf August (no holiday for me!).
But my 'Teaching & Scholarship" colleagues have very little to do over the next couple of months, bar the usual teaching admin & prep - which I do as well. However, during term-time, they teach more hours per week than I do.
As I said in my first post ...
Think about it this way: most university teaching terms total around 24 weeks per year. Then there's the exam period, so say 30-32 weeks per year. Then there's 4 weeks annual leave (ha ha, as if we get to take that, but anyway)
So if you're not doing research full-time (or to the time of your contract) in the other 16 weeks of the year, then your work load in the actual teaching terms is necessarily higher.
So that your workload over the entire working year averages out to 18 hours per week.
This will necessarily mean you do more than 2.5 days per week in the teaching term, because during vacations, it is unlikely you would be doing anywhere near that much, apart from marking and any administrative work you're allocated.