Sump bolts

Hi All....trying to change sump on disco 200tdi - the bolts had been coated with sealant last time they were tightened, with the result that they have been bas***ds to get out. Hammering a 3/8" socket onto them sorted out most, but three will not shift, and have just rounded. Is it safe to apply heat?
Cheers....

Success! Heat, angle grinder, mole grips, hammer, selection of sockets, screwdriver....blood, sweat and tears...but..yes! it is off!