Jam them up with a screwdriver to stop them spinning (the normal prob) and use a cobalt drill bit, always start with an 1/8" (3 or 3.2mm) and then open out. A 5.5mmm should do the job, if the hole is thro the middle and haven't a bigger drill, they'll then shear off with a good blow from a chisel..
We all seem to getting thos washer tanks out around now, did mine a couple of months back, but cannot get those fancy anchor nuts anymore from VW... but Merc parts do somehting very similar, M6 and nicely finished. I used some deep inset anchor plates from local motofax, and hunky chunky self tappers to hold it back up there, bit of a game though
Alwasy pays to have wide range of fasteners around, apparently some garages have 'a man with a van' comea round monthly or so, just selling specialist motor industry style fasteners - like a Snap-On style franchise from a company in Kent that supplies them all I was told.
Doubt if you wanted to know all that
