The bolts for the slave cylinder are accessible but awkward from the engine bay especially if rusted solid (spray from road wheels) heat, plusgas and persistence will help greatly stubby 13mm spanner on the under side and socket on the top. When you have rotated the top bolt enough to lock the spanner aginst the bracket maintain the force and you can then get both hands on the socket to apply proper force. Grinder/dremmel is the last ditch solution and great care must be taken not to damage rubber hoses in the area. Some time spent putting tin foil over hoses to protect from heat or gaffer taping some hardboard may prevent further tears, on petrol engines be very aware of fuel lines in the surrounding area. When replacing the rear bolt which is harder to get to, first loosely fit the front bolt and nut from under the van. If you don't have access to a second pair of hands then use a strong magnet placed on top of the slave and bracket in place, as you push the bolt up from below, the magnet will hold the bolt in place while you get to the engine bay , pass your fingers to the underside of the bracket and hold it in place while you remove the magnet then get the nut and washer on the top, 13mm M8 bolts 30mm long are required 8.8 tensile strength recommended.