Ian, sounds like a very tricky job, small corroded bolt heads, awkward access, seized threads etc. etc. One of those jobs that requires a bit of touch and feel for whats happening to the bolt. As has already been said, will require penetrating oil, if you can get a clean hit directly onto the head of the bolt this can help break the thread seal, then working the bolt back and forth to get it unscrewed and of course.... lots of patience. I always pick the most awkward one to start with. Make sure whatever socket you can get to fit is on tight and square. If you have to drill out a sheared bolt you will have to be very (very) accurate. I'm not familiar with the kit you mention but the drill should at least be a cobalt one as they will be HT bolts I would have thought. Good luck.