I read recently that AA guns had been in action atop the R.N. Torpedo Factory in Alexandria at the time of the Clydebank Blitz . I don't know if this is true or if it has been posted already.(I searched, but didn't have any luck!)

Although I know the Argyll factory well, and much of its history, I can't say I've ever come across anything to substantiate this story. Unfortunately, since the building is all but gone, apart from the façade and the wings immediately behind, it's not possible to go and have a look at the strength of the roof. But, it was built in 1905, so no likelihood of planning for this use n advance, even though it then cost over £220,000 to build.

At a guess, I'd say the roof over the production area (almost 24 acres) would have been too light to support anything, and the pitched roof over the admin section would not have suited.

I suspect there may possibly have been some machine gunners posted on the roof to discourage possible low level attackers, and the story has grown a little over the years.

There could well have been LAA (light anti-aircraft) batteries on the site, but these would have comprised light guns and possibly searchlights towed into place around the site. Hoisting them onto the roof and probably having to reinforce it would seem to be an unnecessary effort for little advantage, bearing in mind we are considering guns intended to lob shrapnel into the sky. Anti-aircraft guns were meant to fill a box in the sky with shrapnel and deter bombers from entering, with the intent of disrupting their flight, not to shoot them out of the sky in the fashion of most Hollywood productions. Although LAA and smaller could try doing this, as they couldn't toss their shells so high, they were firing at closer, lower targets.

If you have a look at the AA Batteries Clyde page, and home in on Alexandria (say the Bellsmyre battery) you'll see that the HAA batteries were pretty close, meaning that an AA battery on the roof would not have any advantage.

You might have access to the info somewhere else, but if I recall my reading of the Blitz, the AA fire was sadly ineffective, and only one bomber is attributed to it over the whole period of the raid. I should make a note next time I come across this.