My Mojo failed its SVA on inadequate demisting. I was using a centrally mounted axial demister. There were 2 issues. Firstly it was deemed that there was inadequate fan assistence. Secondly there was not enough coverage of the windscreen. Beware of ads for products that purport to be SVA compliant, they probably won't be if your car is being tested at Southampton!

Here's my solution
2 X 12 volt 160 watt hair driers from www.towsure.co.uk @ £8.75 each.
2 X 70mm rocking vane dash vents from nfauto.co.uk @£9.95 each.
1 X 50cm length of 50mm aluminium heater conduit again from nfauto.
1 X 30 amp switching relay.
1 X blade fuse holder and 30 amp fuse.

There is ample room under the Mojo dash to mount the 2 driers end to end. I mounted a plate between them to stop the opposed fans from cancelling each other out! The 2 driers together will draw 27 amps, too much for most switches. I ran a feed wire from the ignition cutout switch through the 30 amp fuse to the relay which was wired in the usual way.
I removed the handles from the driers and bolted them to some aluminium angle. Some fettling of the plastic casing was necessary. I then made up a plate out of 16g aluminium that is bolted to the underside of the dash and the angle that holds the driers.
I used a hole saw to cut the holes in the dash top for the vents.
The aluminium conduit can now be used to link the driers to the vents.

A word of caution, 27 amps will flatten a battery very quickly! Only use the demister with the engine running and ensure that your cabling can take 30 amps.