How much damage does dropping a 1000-pound flagstone on someone do?

Probably like 10d10. 

That's a pretty good round even for a level 9 character right?  I guess that is the payoff for arranging to have a fight somewhere with 1000-pound flagstones lying around.

Would you say a Dex Save negates the damage or would it be a Spell Attack to hit?

Since the slab is in free fall I would say a Dex save. 

It’s essentially a more expensive catapult spell.

Can this be used to move yourself about?

Quote from SyllinGalanodel >>

Can this be used to move yourself about?

 As long as you are a huge or smaller creature, yes absolutely. It does require your action, however, and RAW you would be restrained. Probably best ask your DM if you want to houserule it so you're not restrained, I'd certainly allow it myself.

 If I were to use Telekinesis on a creature and thrust them into a tree that was 30 feet away, how much damage would that deal?

Well the creature moves at about 5 feet per second which is about 3.40909 MPH, and the average walking speed of a person is 3. MPH, so the amount of damage you would expect if you walked into a tree. I would assign improvised weapon damage, 1d4, maybe 2-3 d4 depending on the circumstances

Could I use telekinesis on someones big toe to give them a charlie horse? (Just wondering)

Quote from Drayze_Money >>

 If I were to use Telekinesis on a creature and thrust them into a tree that was 30 feet away, how much damage would that deal?

 An easy house rule for that is to equate it to falling damage. So 3d6. 

There doesn't seem to be anything discussing the use of a weapon with telekinesis, But surely that would fall under, "You can exert fine control on objects with your telekinetic grip....", trying to homebrew something for it, but not sure if it should just be a spell attack oars if the caster is wielding the weapon themselves?

Yup, I think it would work. Fine control, however, would also mean you have to be proficient with it as usual. So a wizard wielding a longsword with telekinesis is still a wizard using a longsword. 

The spell allows you to change the target at any time, removing any effect the spell has on it.  So you can move yourself, and then release yourself.  If you are not on solid ground you might be subject to falling damage if you do not have a means of flying or hovering after removing yourself from restraint.

If all the bones in your body are broken can you use this to control yourself

Recently my group has been using this in place of a Hold Person spell. Does that work and is it continual athletic checks to break?

Quote from Dovenwolf >>

Recently my group has been using this in place of a Hold Person spell. Does that work and is it continual athletic checks to break?

 So, it does what it says. It does not paralyze a creature in the same way that hold person, it restrains them, the conditions are different in that a restrained person can still act and doesn't get auto crit like paralysis
It's also a little different than escaping/sustaining a grapple. First off, it's a straight strength check versus a straight spellcasting ability check, that means neither side can add proficiency (unless they're a bard with it through magical secrets, they get half proficiency), and second, every turn the caster has to use their action to make their check, if they don't use their action, they drop the creature. The creature captured by the telekinesis can't use its action to try and escape, they only get out if they beat the casters check, or if the caster doesn't use their action to keep holding them

are you able to move a creature on the same turn you canst telekinesis on it?

a warlock in my group would use it on his shield and then sit on said shield to move himself about

Can you rip of someone’s limbs with this spell, like if someone had the hand of vecna, could you hypothetically tear it off insta killing them