In reply to Papillon:
if they will do the mitre at a reasonable price - do it. they will do it much better than you can.
don't forget to seal the edges of both the old and new boards.
scrape the rough stuff off the old board then seal both by forcing a thin smear of silicone into the rough chipboard.
i advise sealing the back edge of the board in this way also.
when you put the mitre joint together add another squirt of silicone to the mating surfaces and push the joint together.
tighten the mason bolts and then carefully wipe any excess silicone from the worktop surface.
depend what sort of sink?
sit-on or inset? ie. will you see the cut edged of the worktop that you have cut out.
most inset arrangements are for timber or stone tops. i assume you will have a sit on sink?
use the old worktop cut-out as a template. mark the new worktop and cut out with a jigsaw.
the edges don't need to be perfect if you cant see them!
vey important to seal the edges as above and also add a line of silicone on the surface of the worktop around the edge of the cutout to provide a seal between the worktop and the sink top when you lower it in.
notes:
1. jigsaws also cut on the upstroke. us a fine blade on high speed. it's best to turn the top upside down (and your template top) and mark and cut from the bottom).
2. routers can be used to do sink cutouts. you have to do multi-passes to cut through the depth. it's only necessary if the edge is to be on view and a template and guide wheel are recommended.
3. dry fit everything first before adding silicon - just to make sure it all works.
4. you can get coloured silicon - particularly useful for very dark worktops etc. you don't want a white joint line in a 'granite' top.