Rubbish, It's rampant. I was looking to buy a mk8 Honda Civic in LAte December / Early Jan and enquired and looked an at least 7 before buying. Caiught 3 people out ( 1 "dealer" and 2 private) who where dumb enough to have for sale mileages less than recent previous (or several previous) MOT's recorded on the Goverment website.
How, it was easy, took laptop and blackberry (tethered as a 3G modem) to view the cars and after the usual questions and when looking at the paper work plumbed the details needed off their V5C logbook into this website infront of them and compared the results with milage recorded to what thewy had on the ODO and what the service history said.
All three had huge mileage missing past the last MOT and one past three MOT's ago. When you pull them up on it they go red and say they were not aware!!
Yeah right. Several others too where dodgy as 1/2 of certain years had only covered a few hundred miles when the first six months they dod 10k or more.
People are just lazy and stupid most times and many many people still don't know about this digital online MOT check. Why do you think these days several ebay / Autotrader adds have the licence plate number photoshopped out? This alone is not enough, but a quick call to get a MOY number or V5C doc ref number and you can start checking out mileage history before even leaving the house.
Also is good to know that BMW, VW and Honda all keep "central" service history records, so it matters not what main dealer in the country did the work the mileage and work carried out records are all stored against the cars reg number and can be pulled up and checked at your local stealership, oftenn with just a phone call to the service dept.
You don't even have to pretend to be the owner, jaust say you are looking to buy and want to maintain the service history withg them whaen was the last major service done etc, then after that they will tell you the date, mileage and what type of service or warrenty work was carried out.
Jot this down, compare to the online MOT records and make an assessment. Best you can do i'm afreaid. Works best with 05 or 06 plate car's as they just turned 3 years old when MOT's went didgital and online so have the most of online history to compare and spot the fiddlers.