I work Mon, Tues and Weds as well - 5-10.
When you apply for a specific job they should tell you the hours.
I find if you are willing to work Friday/ Saturday they will be more likely to take you as they are the busiest days.
Changing your hours depends on a lot of things, how many other staff there are and the times or 'business needs' as they call them..... it also MASSIVLEY depends on your manager wether or not they give you the hours you want.
For example, we recently got a new Grocery manager, he runs a much tighter ship with hours, but he created 5 new jobs for Fridays and Saturdays the first week he came - so he did help out in that respect!
I've work at Tesco PT for 5 years and have worked on 3 departments, all sorts of hours and have changed my working hours at least seperate times.. I have been contracted to work everday of the week at one time or another... I would avoid Fri and Sats tho as they are the busiest...