Student housing hell

I need help!
Basically, me and four friends are moving in together for third year in September. We spent months looking for property’s with most being snatched up before we’d even had chance to look around. We finally found a place that we thought was perfect, during the viewing the agent told us about a summer retainer period which at the time she made out that this was an ‘optional’ thing. She also said to us how she had loads more groups looking around the property and if we wanted to secure the property we should sign the contracts immediately. Obviously with this pressure and the lack of property’s around, we all signed the contract and didn’t think anything of it. I was always under the impression our tenancy was 42 weeks long with an optional summer retainer of 5 weeks if we wanted to live in the property over summer or store any of our possessions there. We got an email off of the company saying how they were excited to welcome us blah blah blah and I decided to have another look at our contract just to check I had enough student loan to definitely pay for it, through closer inspection we are being charged 5 weeks of rent for a summer retainer period even though all of us thought this was optional. Thinking this was a mistake I emailed just to double check that we weren’t being charged the £650 pound for the retainer, to my surprise they came back saying this retainer was not optional and the retainer fee would be added to our rent schedule. After they said this I read the contract fully and it says that we will be charged this money regardless and the optional part was if we wanted to leave our possessions in the property. Below this in bold red writing it says this does not guarantee the property will be ready for us to leave our possessions. This summer retainer period started on the 1st of July, they told us that the property is not ready, so we are all paying £650 worth of rent for none of us to be able to occupy the residency or leave our possessions? So we are paying a 47 week contract and only 42 weeks of those 47 are we allowed to occupy the residency. I searched the legality of this and it says that it isn’t legal, although this was not made clear by the landlord or agent, we have signed a contract, im just wondering if anyone knows what we can do as we are all confused and annoyed. Thanks! (also really sorry if this doesn’t make sense, it doesn’t make sense to me either).