I just spoke with a very helpful chap from VOSA and asked the question if I am towing in our case a large flat bed trailer with floats for either sale, hire or on loan for demo would we require a tacho.
We have two tow vehicles a Mitsubuishi L200 and a Dicso 3, so the gross vehicle and trailer (when empty) combination would exceed 3500kg. Obviously when the trailer is loaded we are pushing 5000kg.
His comments were if we are using the trailer for business then we would need a tacho fitted to our vehicles.
I then asked ok so if say I had driven for 9hrs (in two 4.5hrs sections) then decided to drive home having delivered the goods towing home an empty trailer, would the drive home be classed as personal use or business use as technically as self employed I personally decide to drive home then in "personal time" what would the outcome be if stopped? His comment was "well thats a grey area"
He also said that Disco 3 are very hard to find a tacho for if they have one as they have had a lot of issues where tacho's have not worked properly in those vehicles and they have had to keep a "Manual Record"
Anyway hope that helps