Enforce the payments from Nadine and Billy

Nadine is having a new log burning stove fitted at her house, but she first needs to renovate her fireplace. She contracts with Albert, a builder, to take out the old gas fire and gas pipes and fit a new stone lintel over the hearth ready for the new stove to be fitted. Albert quotes a price of £1,100 for this work and they agree that it will be completed by 15th December.
On 4th December, Albert telephones Nadine to explain that he will be delayed because he has fallen behind on other jobs, as he had to
quarantine with Covid. Consequently, he will need to reschedule her job for after Christmas. Nadine tries to insist Albert carries out the work by 15th December, because she has the man who is installing the stove, Billy, booked for 16th December, and she wants all of the work finished for the Christmas holidays. Albert explains that the only way he could do it would be to take on extra staff, which he cannot afford to do.
Reluctantly, Nadine agrees to pay an extra £200 if Albert completes her work to the original schedule.
Billy hears about Albert’s potential delay and contacts Albert and promises him £50 if Albert gets his work done by 15th December. Billy does this because he is working to a very tight schedule on other jobs and does not want to fall behind.
Albert takes on a labourer for a week and works long days on his jobs and completes the Nadine’s work by 15th December. Billy then fits the stove on 16th December.
When Albert asks for payment, Nadine says that she will only pay the originally agreed £1,100, saying that Albert has not done anything more than he originally agreed to do. Billy also refuses to pay the additional £50.
Advise Albert whether he can enforce the payments from Nadine and Billy.