Pete's got it right. I use a Z shaped screwdriver. The screws come of pretty easily. The regulator comes out easily too but can sometimes be a bit tricky to get back in, especially if it has new, long brushes. I find if you tilt the VR back a bit and give the brushes a chance to seat properly as you push it into position, it's a piece of cake. You'll need two hands, one to hold the regulator with the brushes slightly compressed, and another the thread the screws back into place, unless you're hands are a lot more nimble than mine.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold