Screwfix do a stuff called "silicone eater" which softens it ..bit messy but best to remove as much as poss. with sharp knife/scraper/chisel beforehand. They also do a "silicone sealant remover" and a "sealant remover" Take your pick.
I agree with the others re silicone removers although I have not used it myself.
Petrol is just a tad too hazardous for my likeing for the impact glue remover. If there is much to do get the proper solvent for it which will be toluene or xylene (aka methyl benzene and dimethyl benzene respectively) I recencently removed some using antifouling thinner, which was trimethyl benzene, because it was all I had to hand at the time.
Hammerite thinners may do but the fomulation is different now compared to what it was a few years ago.