If the deposit has been repaid, can overpaid rent be kept?

Campaigning and Policy

Long story as short as I can make it, I am a Landlord, but until last week was also a tenant. This relates to me being a tenant but is good knowledge to have in my landlord business.

I originally gave notice to vacate the property on the 5th May, being the date I had paid up until. My landlady asked if I could move out on the 2nd to accommodate a new tenant. She confirmed via text that she would refund the 3 days rent.

The day before we were due to hand back the keys (the 1st) she entered the property and found some dust on a light switch and a missing lightbulb.

On the 2nd, the (very professional) letting agent visited for the inventory and happily wrote a cheque for the £1,200 deposit.

I know I am biased but believe me when I tell you the place was spotless. The oven, fridge, shower, washing machine drawer... and carpets had been cleaned. Any marks on the wall were touched up.

The landlady is now holding on to the £110 rent we overpaid. Given the letting agent was more than happy with the condition and returned the deposit, what legal right does she have to retain the extra rent?