Prices are rocketing on sites so this has a knock-on effect in the domestic market. Expect to pay upwards of £120 per day.
A brickie may take three days on his own to build a 750 brick wall depending upon complexity. If it is a straight run with no pillars, perhaps less.
If there are pillars, soldier headers, wonky foundations or stepped foundations, bricks to barrow etc maybe more.
He may want to give you a price, i.e. £500 all in, the hit the job like a mad man. Start a daft o'clock and finish when it's dark and get done for two days.
Or he may agree a day rate and plod along, 8:00am till 4:00pm, and take a leisurely five days.