I'm not sure what you mean??

Surely a weed killer should have been applied before the driveway was laid?

Do you have any reason to think that weeds are going to emerge through your Tarmac?

I am with Teegee, weeds do not grow out of macadam, they grow through it if there are gaps. The heat of the laying will kill most weeds. I cannot think of any native UK plants that will grow through! I do not think even Japanese knotweed would grow through it from below. Save your money and do not buy any weedkiller.

Spraying weedkiller on a hard surface like a tarmac drive would be frowned upon. There is a real danger that the weedkiller will be washed into surface waterdrains and then into the water environment. Any treatment of this sort must be licenced which is why the weedkiller is not available for public use.
I also agree with the above comments. Why?

I agree with the others. A properly layed Tarmac drive would be impervious to all but perhaps japanese knotweed

It should be impervious to water and so spraying weed killer on it wouldn't work as it would just run off

My husband used to be in the line of work which laid such drives as these.
He tells me they should have used a correct weed killer before the drive was laid. Furthermore, if the drive was dug out correctly it should have been deep enough to remove most weed roots. If not, and correct weedkiller wasn't used, Docks and Knotweed will come through. You pay for what you get I'm afraid.
Weedkiller applied now will only run off and may cause environmental problems. :(

Thanks everyone for the advice which has a common theme. I shall sit back and wait till Spring in the hope that weeds do not emerge.

Hi Bkerridge. Don't sit on your butt till Spring. Every day is a gardening day!
I agree with all the other comments but you should quickly inspect the drive every day and if you see ANYTHING green poking through, pull it up. That way you will kill off whatever it is.Or, keep a spray spot killer handy and give the green stuff a quick dose as per the tin.
Southportsmith

Quote; if you see ANYTHING green poking through, pull it up.

I wouldn't!

I would get the people who laid your tarmac to come back and remedy the problem.

If you pull up the weeds then you have no proof of shoddy workmanship!

Fully agree with Teegee on that!!!!!