Going round the M25 the other day as a passenger I noticed that a lot of foreign lorries had plates on the trailer different to the ones on the tractor unit,does anyone know why this would be ?.
It’s because trailers in Europe are registered as separate items so have their own number plates
because their trailers are registered and have their own number plate .
As above. The UK is the exception rather than the rule.
Some countries show only the trailer number on the rear (France, Germany) some should show both trailer and unit numbers (Spain, Italy).
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I used to run around with 3 different plates.
One for unit, one for trailer, one for kooiap/moffet.
Had a good argument with a young, keen (and up himself) VOSA kid once who… ‘‘Did not think it was right’’ 
I handed him a bit of paper with Rue de Loix Bruxelles written on it and told him to tell them that HE thought it was wrong.
He said ‘‘Is that your depot address’’ I said ‘‘No EU hq’’
It DID happen before anybody has a go. 
In France you will find trailers pulled by cars/vans, that are above a certain weight (PTAC) will have their own registration documents and reg plates too.
< 500kg same reg as car
500kg own number plate and own registration papers
750kg will also have its own insurance policy
Bulgarian trucks also have 2 number plates…one for unit 1 for trailer…because they are classed as 2 vehicles for registration and insurance.
Only having one plate that covers both causes us a fair few problems going into Russia . More from the Latvians than the Russians tho
The Spanish seem to have two on the trailer, one white one and one red one.
Radar19:
The Spanish seem to have two on the trailer, one white one and one red one.
The red one has the letter “R” for rebolque or trailer. The white one is same as the unit.
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Franglais:
Radar19:
The Spanish seem to have two on the trailer, one white one and one red one.The red one has the letter “R” for rebolque or trailer. The white one is same as the unit.
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The Italians the same, different coloured plates on the trailer, the one with an “R” on it for “rimorchio.” usually yellow, the white one is the same as the unit
Same here, separate rego.
Some Irish ones have different plates too but that’s for a different reason now… 
Thanks for all the answers lads [keep your elbows in ]