Where can I get brick acid?

That's a common trick at Travis Perkins, if they don't stock something it doesn't exist, only because they want to sell you their stuff.

I got some only a few weeks ago so it is still made, try small hardware stores, mine was about £4 for a gallon, but don't expect miracles, acid will only usually remove alkali type stuff, so general discolouration of bricks will not be affected, although algy and mortar stains will.

Do it *before*

I don't think brick acid would do a lot of good in this situation - a pressure washer is probably a better bet.

They do atock it, or at least they did a few months ago the last time I bought some. Travis Perkins Mortar Cleaner - 16% hydrochloric acid.

You definately don't want brick acid. The only thing it will clean is the mortar out from between the bricks, which is a really bad idea. The dirt up there probably isn't susceptable to removal by acid, as rain would have washed it off. I'm afraid I can't think of anything you could simply pour over without also some surface scrubbing with a soft brush. Beware that chimneys were often built with expensive facing bricks, which can be very soft and easily damaged by hard brushing.

Agreed.

Standard issue for this is sandblaster or pressure washer.