Caravanning Now

The regulations that govern the use of towing mirrors is paragraph 33 of The Road Vehicles (Construction and Use) Regulations 1986, which were replaced entirely by SI 3165 of 2005.

For cars first used after a certain date in 1978 but before 26th January 2010, one mirror is required on the offside of the vehicle.
So an offside towing mirror is required if the standard door mirror doesn't give a view down the side of the trailer.
If the interior mirror or other device (such as a reversing camera) does not provide an adequate view to the rear, a nearside mirror is also required.

For cars first used on or after 26th January 2010, mirrors are required on both sides.
So if the standard door mirror doesn't provide a view down the side of the van, two towing mirrors are needed.

Also the amount by which the mirrors may project beyond the widest part of the outfit depends on when the car was first used.

For current outfits, mirrors may not project more than 20cm beyond the widest part.

For cars first used after the 2010 cut-off date, this is increased to 25cm.

All mirrors fitted to any car first used on or after the 1978 cut off date must conform to specified EU Directives or to ECE Regulation 46.

However, the earlier Directives provided routes to conformity that did not require marking.
Provided you bought your mirrors from a reputable dealer, you should be able to use them on any current outfit no matter when the car was registered.

For cars first registered on or after the 2010 cut off date, the mirrors must comply with a later Directive or ECE Regulation 46, and both require the appropriate markings.

The type of mark signifies whether the mirror conforms with a Directive or the ECE regulation.

Mirrors that conform with a Directive are marked with a rectangle in which is a lower-case "e" followed by a number;

Those that conform to the ECE regulation have a circle in which is an upper-case "E" followed by a number.

In both cases, the number signifies only the country in which conformity is claimed (and thus usually the country of origin) and where 1 = France, 2 = Germany, 3 = Italy, 11 = UK, etc.
So (using square brackets to signify a rectangle and round brackets for a circle), "[e3]" signifies tested in Italy and conformity with a Directive, "(E11)" signifies tested in UK and conformity with ECE Regulation 46, etc.

In a nutshell, any mirrors you buy now from a reputable source will be completely legal for use on your current tow car no matter whether those mirrors are marked. However, if they're not marked you will not be able to use them lawfully on any car that's first used on or after the 2010 cut-off date.