If you are prevented from entering the property on the day the contract says the tenancy begins that is a breach of the contract.
If you suffer a loss as a result of the breach, you are entitled to be restored to the position you'd be in had the loss not occurred. For example, if you had additional removal fees and a hotel bill resulting from this breach, you'd be entitled to claim those costs. Generally the landlord would be liable for this.
It is no defence that the agent is unavailable to give you the keys - they should have accounted for the day being a Sunday or started the term on a day when someone would be available.
I suggest getting in touch with the landlord in the first instance, civilly explain the situation and ask if they can arrange for you to enter the property on the given date - they might not know what is going on. Keep a log/diary of your communications with the agent and the landlord. Make sure you have a plan B for somewhere to stay. Call Shelter too, for free advice.