uk land law - contract law

Donna emailed Mo, a brickwork professional, for a quote for the restoration of the faade of her

house. Mo text messaged back: "Can start in two months. Price to be similar to the work I did on

your holiday house last year, but we'll agree as we go along. Please let me know so I can book a start

date". Donna was on holiday and did not see Mo's text, but phoned him three weeks after, as soon as

she returned, to say that she is happy to proceed. When the works started, Mo told Donna that any

bricks that need replacing will cost 12, but when he saw how nice Donna was offering him coffee

and biscuits every day, he said "You know what, I will only charge you 10 per replaced brick". After

the works have been completed, Mo invoices Donna for the full amount for brick replacement.

Advise Donna.