Well I got it done, eventually!
Thought it was just worth posting up a picture of what I cut/did, for the benefit of future folk tackling this job the 'cheat' way.

This is a view from offside to nearside, just in front of the pedals in the driver's footwell. The picture in Haynes didn't make this sufficiently clear to me, and I started off by trying to access the matrix by dismantling the heater control panel and all its layers


. It wasn't 'til I thought about seeing, on the inside, where the pipes came through the firewall that I realised what a plonker I was being.
On the LHS of the picture you can see the vinyl dash cover trim that I've cut and peeled back carefully from the underlying crunchy-foam-covered plastic that makes up the dash. That vinyl is easily torn, so take it very steady if you want it to look OK later.
You can see I've cut a fairly large area of of the dash material away, using a hacksaw blade, making sure there was no wiring or anything else important behind where I was hacking. I also cut away some of the trim from the portion in the lower left of the picture (under the L-R orange arrow), but you can't see this bit as the vinyl is back in position.
The orange arrows show the 'steering column bracing strut' ( at least I think that's what it is, Haynes mentions moving such a thing) and how it has been undone from its original position (orange dot or thereabouts) and rotated toward the rear of the car just enough to clear the back edge of the matrix. Some gentle persuasion required to get it this far...
The green arrow points to the bottom left corner of the fuse/relay panel part of the dash, which I had to cut just above that hinge point, and then pull forward and up forcibly with some string to give enough clearance for the old matrix to come out and the new to go in. I had hoped that once the matrix moved out a little, it might be possible to tilt it downwards to clear this gubbinry, but that wasn't to be. It had to come out horizontally, all the way.
Yellow arrow points at where the single fixing screw is/was that secures the matrix.
Blue arrow points at the rubber end cover that came with my new matrix. It was subtly different from the one on the original matrix, and wasn't a tight fit on the pipework, so I pre-installed that at the end of the slot before sliding the new matrix in.
Note: Before you fit the new matrix, don't forget to mop out the coolant that has accumulated inside the air distribution unit (about 30 sheets of kitchen-roll's worth...)! You may wish to recruit someone with smaller hands/arms for this if you can find a willing volunteer.

I think Haynes suggests removing the blue control cable just below the yellow arrow, but I didn't need to.
I'm afraid I couldn't be bothered to put all the pieces of the jigsaw back properly, as the vinyl sits in position quite fine without anything behind it, and the fuse panel seemed secure enough for my liking even with the cut I had to make.
Thanks to all for the help and suggestions.

No more steamy windscreen, yay!