It's usual to wire in 2.5mm twin and earth from a 16A MCB at the consumer unit to a fused connection unit fitted with a 3A fuse. Then 1.5mm heat resistant flex from the FCU to the boiler.
You could wire the FCU in 1.5mm from a 6A MCB to the fused connection unit, but because the FCU could be capable of being used in the future for an appliance rated upto 13A (unlikley thought that may be) using 2.5mm would be good practice.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local