what is the best way to take off a sump bolt thats been torqued too tight on car?

Not necessarily put on too tightly but with the road dirt and wet over the last 6 to 12 monhs this can affect the bolt.

Depends how much time you have, some penetrating oil or similar (even wd40 or FT85) around the bolt and left for a while - keep applying it, this can wash out some much and lubricate the bolt a bit.

After that get a ring spanner that is close fitting (remember that some brands of cars still use imperial bolts, som metric and get the wrong spanner there is just enough give in it to round off the bolt head). extend the leverage with a piece of pipe along the whole spanne handle length and gently apply pressure to it - sudden pressure and spanner falls off rounding the head a but more.

If the bolt head gets rounded you could try the stiltsons mentioned before or also you can get a socket that sits on the bot and as you try to undo it the socket digs into the bolt gripping it tighter and tighter and these can help.

For a one off job consider if the extra tools and hassle are worth it or paying the mechanic to do the job (better if the bolt is getting damaged)

If you get the bolt off then replace it with new one by routine and a new washer. Also make sure you wipe around the inside of the sump plug hole just get all the rubbish out of that.

If it is rounding off pop into Halfords and buy a new one or you will have difficulty forever. As said you must use the correct size socket or you will be wasting your time. A short piece of pipe to extend the ratchet should give you ample leverage. Some sump plugs have a hexagon socket in them too. Have a close look or press a piece of paper on to get an impression. Again you will need a tight fitting allen key and an extension tube. If all fails with the socket and allen key the last resort is a Stillson wrench. These have serrated teeth and grip tighter as you apply force. It will bugger the hexagon but the sump bolt will undo. Guaranteed. Hence the new sump plug needed.

The link shows a selection of Stillsons. If Toolstation are not near you maybe Screwfix will be.

P.S. New washer will be needed. A new sump plug will come with a washer. A dealer for the car will probably be the place to buy a new washer if you re-use the original sump plug.

Impact socket (The correct size exactly) and a medium breaker bar as there is no need to go medieval on it and then constant pressure should crack it.

Remember once it is off you will need a new crush washer before you replace it and find the exact torque it needs setting to. Older cars used to be pinch tight and half a turn, but some modern cars need a specific tightness.

You'll need a 6 point socket or wrench, tap it on and angle the wrench handle backwards toward the bolt head so that the torque vector is directly inline with the bolt head. This way, when you apply pressure it won't want to walk off the head.

extend the arm of the socket spanner with a metal tube of some sort for more leverage. The socket

has to fit the nut snugly or you will be rapping your knuckles.

At least a ring-spanner, if not then a socket, and more muscle than you have available/longer lever.