Before I pay said man £50(?) to do this is there any point in me
buying these foam or powder destroyers that are available.

A good coating of Nippon ant powder on the horizontal surfaces of the air
brick will do the job. £2.50 from your local £1 shop (eh???). Wait for a
cool part of the day, give the brick a good blast. Sorted.

The foams are OK, but the powder will poison the entry/exit point
effectively. I've only used the foam once, when it was impractical to use
powder (wasps entering up a verical wall), but it did the job. Expect to
pay £3.50 from your local £1 shop (eh again???).

Al.