The plan was for a 20" reflector mainly for use with a CCD camera.  I choose to start with a 1" thick plate glass blank, mainly on the grounds of cost.  For the sake of the CCD chip and to keep the size of the scope down I went for a focal ratio of 4.   The sagitta depth required it a horrific 1/3 inch.  Fearing I might spend the rest of my life hogging out I took a look though the ATM list archives and found a few threads and sites detailing the use of diamond cutters for rough grinding.  Ricardo Dunna's and Peter Smith's  sites describe what is possible but the methods require a mirror making machine or a drill press. I have neither. The TRO site is a good description of the low-tech approach of pivoting the cutter at the centre of curvature.  I adopted a similar method but choose to suspend the cutter for the roof of my garage.

So starting at the top.  Here is a very poor picture of the top pivot point high in the roof of the garage.  The pivot is a ball and socket affair while the top of the arm is 1 1/2 " square wood

Control over the depth of cut is by bolt arrowed.  I welded some washers into the top of this joint to help stiffen things up.

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This is the cutter in action.  The point of contact on the glass is directly under the pivot arm.  The brick acts as counter weight (its actually bolted to the horizontal rod, the tape being insurance).

Another picture of the disc.   The disc is from a tile cutter and cost £10.  The spindle is a bolt and 2 nyloc nuts keep the disc in place.  The rubber backing of a drill sander attachment is used behind the disc to provide support at the edge to the disc and acts as a clutch.  The spindle bolt was pressed into a bearing to provide more support.

After ~12 hours worth of cutting this is the result.  The marks on the surface aren't very deep and should soon come out.

Pros: I am really happy with the results.  The speed with which the cutter removes glass is amazing.  The bulk of the glass was removed in 2 cuts of about 3mm a time. This meant the side of the cutter was doing the work rather than the edge.  Still the diamond disc held up really well and looks good for another 3 or 4 mirrors, even if I'm not.

Cons:  Nothing serious.  Water tends to get everywhere.  Bit of playing around with glass placement required to keep ground dish concentric with blank.

The IMPORTANT bit. If you are thinking of doing something similar remember. i. Use an RCD on the power supply. ii. Use lots of water to disperse ground glass and prevent dust. iii. Safety specs and dust masks are a must