As above give them a soak with a penetrating oil. then just a case of going slow with an even force to get them out. Try not hitting the spanner with a hammer to get them started. We have all been there with a half bolt stuck some where. Best solution is try to drill out with a left handed drill bit and an easy out. Worst case scenario. Drill all the way to within a 1.mm of bolt diameter them hammer out with a needle punch. You can always get them out just a case of time.