I reckon I have found the source of the dastardly leak.

1. I have just spent the last 2 hours pulling out vacuum hoses under the hood, and blowing through them just to see if they leak. Despite the rather grotty nature of these original Volvo vacuum hoses, they've held up well.

2. I finally blew through the hose that leads to the vacuum reservoir (as you guys have pointed out, the reservoir tank is hidden away behind the bumper) and detected a leak.

3. I pulled this hose out completely and blew through it. No leak. Hmm, might it be the reservoir tank itself? (the thought of that was quite dreadful to be honest - I didn't fancy taking off the bumper since I am feeling rather lazy)

4. Stuck my hand into the tiny gap to cop a feel of the reservoir and the short outlet pipes to see if I can feel any holes or cracks. Lo and behold, I discovered that the tank has two outlets - one to the hose that goes into the engine bay, and another on the opposite side capped off by a rubber nipple.

It's this nipple cap that's split, letting in air. Ding ding ding!

It's a little late now and all the auto supplies shops are closed. I think when I replace this nipple all will be fine :-)

5. I suspect that this leak is also the cause the stall whenever I start the car from cold. If so, I shall be DOUBLY pleased!

I shall report back!