Every job I've ever worked in has had a 7.5 hour "standard" working day although, as salaried employee, sometimes many more hours are worked.
So I'd operate on the basis that it's a 37.5 hour full time week, i.e. that this job is four full (7.5 hour) days a week. Then you're looking at 80% of the pay range given, i.e. £14921-£16376 pa.
Alternatively it could be a 40 hour week, but then to do 30 hours you'd be looking at five six hour days, which would be an unusual arrangement compared to the one above (five short days rather than four full ones). If this was the case, though, then you're looking at 75% of the pay range given, i.e. £13988-£15353 pa.