most tyre fitting places should be able to get your old locking bolts off, best solution before removal would be to ask the last owner, they probably just forgot to give the key to you, often they are kept away from the car.
I am with rallycinq on the horn issue
I was in same situation a couple of weeks ago the day before my MOT, realising the horn didnt work I took the horn off and dismantled it as I recently went through a massive puddle, although a few drips came out of it the horn seemed fine, however the female spade connector on the vehicle wiring was slack, plus had some corrosion, with it replaced and horn back in it all worked perfect.