I think this is "Tanglewreck", by Jeanette Winterson. It was published in 2006, which fits in with your reading it in 2008/2009.
She goes into a black hole (I think she's sent there by the antagonist(s) by being pushed through a shed?) and there're two weird shapes in there, like "spaghetti", and one of them tells her that the black hole stretches you out and then explains "spaghettification". I don't remember how they survived.
I've done a search on the book's text using Google Books. It's not giving me a full preview on all of the relevant pages, but I can find some text extracts that match. It doesn't let me Edit-Copy, so apologies for any mistakes introduced by hand-typing this:
The guards had bundled him out of the van and pushed him towards a corrugated iron shed.
Did they throw you down here like the rest?
Gabriel told them about the shed, but the Voices knew nothing of that.
"... so much gravity down here that it pulls everything in with it, even light. Even light can't escape this place. No Time, no light, just what they call the Stretch"
"The Stretch?" Gabriel was nervous. "Gravity down here will stretch you like spaghetti"
and on a later page:
That be its name? Black Hole?
NOTE: The book seems to have a female protagonist, but Gabriel appears to be a male secondary protagonist. Perhaps the main character will encounter this black hole later in the book. Also, I can see a longer Google Books preview on the first extract, and Gabriel is hearing this from what he calls "The Shape".
Looking at your other points:
I can remember something about an hourglass that might have been important
According to the blurb as quoted on Goodreads, there's an important item called "the Timekeeper". But it may not have been an hourglass. Again from Google Books:
In those days, the Timekeeper was a pendulum clock with a double face ... It kept time well, and it kept its other secrets well
and there, underneath, was a stairway, and on the third step was a box, and in the box was the Timekeeper.
I did find a reference to an hourglass:
Abel Darkwater went and tapped his hourglass. "When Time comes to an end, you too will come to an end. I am sure you don't want that to happen."
You also said:
It started with a double-decker-bus
The book opens with
At six forty-five one summer morning, a red London bus was crossing Waterloo Bridge
After a strange "wibbly-wobbly-timey-wimey" incident, the bus nearly crashes and is then snatched out of time and existence.
The bus and its passengers were never found. It was the first of the Time Tornadoes.
Finally, you stated:
it may have been set in London
From Google Books again:
... she had a feeling that the house was doing its best to help. Then she remembered why she had come to London in the first place; because there was something important to do. If it was important, it was bound to be difficult.
At some point earlier on, someone says
I would like you to bring the child to London - tell her it is a special treat.