swapping a speedo/odometer - the law?

[missfire]

hi,

does anyone know what the 'law' is if i had to swap the speedo/odometer for another unit..? as regards to...m.o.t. time for example..? when they say....''excuse me sir but...WTF..!?!??''

my 100 turbo thinks its doing 0 mph all the time....not good...

i have a 112K unit that i could put in, the one that does not work has 134K on it and i have tried to fix it, just can't spend any more time on it, so swapping the whole instrument cluster is now by far the easiest option. i have too many other things on the car that i want to be getting on with to waste any more time breathing solder on the dining room table.

is there a dvla form to fill in or anything like that..??

thanks