That's excellent news! Although sorry to hear of Dave's passing.
You'll need two sets of guards: both are rears. Remove all brackets and put little stickies over any unwanted holes.
There should still be a 'shortie' front mudguard; the long mudguard is simply attached to this with a couple of bands made from inner tube.
Not high tech, but best use of available materials and intended to be 'quick release'.
In practice I found I never took the mudguards off, couldn't be bothered, so nowadays my mudguards are permanently fixed, although they have plenty of 'give' in the event of a tumble.
For fitting the rear mudguard there is a tab just behind the bottom bracket. You have to cut a half-inch wide slot up the middle of the guard, about 2" long. The modified mudguard is pushed down behind the tab so that the two 'tails' act to keep it in place.
Once you've done this, where it passes under the seat stay bridge, shaped so the mudguard section fits it, another piece of cut inner tube is used to secure it. Quite tricky this bit, and it's the first part of the fitting process. Hold the piece of inner tube so that each end is over the bridge and pull the loops down. Now the mudguard can be fed though these loops and located behind the bottom bracket as described above.
Finally, there is a tab located at the back of the rear carrier, which should have another inner tube band around it, like the front guard.
The extensions are cut from damp-proof course material, which is available from builders merchants, or I can send you some. I can't remember how I attached them, probably pop-rivets, but these days I use self adhesive velcro. Not much sticks to DPC, so it has to be welded with a soldering iron.
Good luck, and not too much swearing; the process becomes a lot easier when you're doing it three times a day!
PM me, or Graham Wallace. Some photos may be possible.
Maybe we'll see this one on the next Birthday Ride?