I managed to grab a couple of hours to have a go at some flash lines tonight. I haven't been looking forward to this at all, but I can't put it off forever. Other people have done an area from start to finish but I've decided to do the whole car 1 stage at a time.
I started by masking off about 5mm either side of the flash lines all round and then carefully 'shaved' the worst of it all off by drawing a Stanley blade backwards at right angles to the flash line. This gets all but the thickness of the tape off.
I removed all the tape and started with the rear offside quarter panel with some P400 wet & dry (I used 3M paper as I've heard that quality and consistency was far superior to other brands) on a rubber sanding block and plenty of water from a spray bottle. I sanded in the direction of the line and was pleasantly surprised to see it come up quite well fairly easily. The rubber block works well over larger flat or slightly convex areas, but I used a small piece of closed cell foam to get in to the tighter corners and around the radius of the cockpit.
Although I can see that this will take some time, I'm more at ease than I was with the process and the results. I'm still not sure about the engine bay seam though, may even fill it and paint it black.