basically i am looking for a summer job and have found a vacancy working in a shop (part of a chain of stores selling outdoor/camping stuff), however the job is not advertised as a temporary/summer job position. If I were to apply for the job would I be obligated to tell the potential employer that I am only available for the summer? Is it likely that a job that is not advertised as temporary would require working for a fixed number of months?
If it's not advertised as a summer position or temporary job then it is a permanent position they are advertising. They won't require a fixed number of months from you but if they were to hire you they would need to hire someone else when you leave to replace you.
You can apply, but it is very likely they'll ask about your availability and things like why you want to work there and where you see yourself in 5/10 years time in the interview. If you tell them you're just after some money over the summer they are unlikely to hire you because ideally they want someone who is going to work there until they retire so that they don't have to go through the recruitment process again anytime soon.