Bumper bolts sheared

The front bumper on by 200tdi is badly rusted in place and so I have got a new bumper, bolts and plates to replace it. I have soaked the old bolts in WD40 for days but have still managed to shear off two of them. Does anyone know where I need to cut to get them out?
Am I better trying to get a hacksaw blade into the slot in the chassis dumb irons next to the bolts, or just to chop up the old bumper and try and cut in from the front somehow?
Not really looking forward to it either way...

attack them from underneath? if the bumper is junk cut the ends and middle off to give you more access maybe?

I would cut the bumper off just leaving the 4 little stubs the bolts go through and see what you can do from there,

Three of mine sheared too when I attempted to remove them. Luckily, they sheared just near the captive nut so I was able to extract them through the cutaways in the rails. I dumped load of waxoyl and used stainless bolts with Copperslip when re-installing. Sorry, that doesn't help you much though!

You need to cut through the bottom. Last time mine did this I used a hack saw blade in a handle (similar to this: link) to cut the bolt just above the captive plate. Took a long time but managed I without damaging the chassis or the bumper.

Thanks - I'll get the hacksaw blade out and give it a go

In the end I sawed through the rustiest part of the bumper near the centre using an electric saw. Then I pivoted the two ends of bumper around the fixing bolts/plates and they fell out into the dumb irons. The two halves of the bumper came off easily and it was straightforward to fish out the remains of the bolts and captive plate thing. New bumper was a bit hard to line up but once that was done it went on easily. Job done.