Mobile speed camera vans

I got a letter in the post today from police Scotland entitled "notice of intended prosecution" and my offence was to exceed 30mph speed limit. It says my vehicle speed was 41mph. I did see the camera van and know I'm bang to rights. At the moment they are just looking for driver details.

Its years since I was last done for speeding. All my past times I've been pulled over by the rozzers standing at the side of the road with the speed gun. I'm sure I'll be proved wrong but I thought they give you a certain leeway so to get prosecuted in a 30 zone you have to be in excess of 42 mph. In that case I'm 1 mph below this or am I mistaken?

I'm now waiting for the next letter which I presume will be a £100 fine and 3 points or the option to go to a speed awareness course. Now is this not more expensive than just paying the fine so a waste of time as you still need to declare it to insurence?

Sorry if already a thread about this so please move if so. I'm not proud of what I did and know that speed kills and all that but it was a tiny village in the middle of nowhere and I'm sure hundreds more were caught out like me by this cash making scheme against us already beleaguered drivers.