Renewal fees are really up the agency. They can charge whatever they want- as long as the client swallows it. Check you tenancy agreement- does it mention the renewal fee? You can ask for a copy of the original application form and check whether they have advised you about these fees before but unless the form says something helpful like: "these fees are one-off only" there is not much you can do. Fair? Nope, but business is business.
You can try to negotiate with the agents- keeping the landlord in the loop. But because you have already paid this fee, I am afraid you will not see this 30 quid again

So all in all- if you want to stay, you are stuck with the renewal fee, as much as we all hate them.
If you are at Uni, the SU there has a really good advice centre- pop in to see if they can offer any further help.