The other thing to remember is that the 3rd and 4th years on such deals cost the same as years 1 & 2. In other words, your getting a better deal on the 2 year deals as the depreciation is higher during these years, than it is in the 3rd and 4th years. So, making sure its the amortised rates that are similar, then years 3 & 4 are "expensive" to be driving a 2-4 year old car, where for the same money, you could be back in a new car. And as said, without the hassle of servicing, MOT's or tyres, most likely.
Looking at GinG's examples:
24 month plan (23 month payments) = £287.09
36 month plan (35 month payments = £272.99
48 month plan (47 month payments) = £272.72
The 24 month plan is around £14 a month more expensive than the other plans. However, the 36 month plan will require at least a service (£300?) and possibly a set of tyres, but lets say you manage to get it back with only replacing a pair of tyres. Still, many modern cars run 18's+ now in their standard spec, so lets say another £200 in tyres. You should get it back before the MOT though, so that'll save £50. But still, that could be an extra £500 in costs to save £14 per month (£500 is £13.88 per month over 36 months). Or, roughly the same cost, more expensive if you end up having to change 4 tyres instead of 2.
The 48 month however, may be looking at a second service if your unlucky, and this would mostly be the "major" service, so maybe £500. And there is a VERY good chance that you will, indeed, need to change the 4 tyres during the deal, maybe even 6 or 8 over the term depending on tyre wear rate and mileage.
Whereas the 2 year deal, you get another new car after 2 years, hopefully at a similar price, and not have any of these "extra" expenses. That would be worth the extra £14 per month to me.