In 2015 the Government published guidance on the carriage of passengers in campervans, the information can be found here: https://www.gov.uk/government/publications/carriage-of-passengers-in-campervans 

Our original article, last updated in 2011, is included below for additional background information.

We asked the Department for Transport for their interpretation on the law as it applies to seat belts fitted in motorhomes, this article gives the essence of the information that they gave us.

The information below relates to the fitting of seatbelts and the wearing of seat belts by adults. There is specific legislation, introduced in 2006, concerning the carrying of children in vehicles. Rather than trying to present all the somewhat complicated regulations here, please see: www.gov.uk/child-car-seats-the-rules

For vehicles built up to October 2007 there was no legal requirement to have seat belts fitted to side-facing seats or seats that make up the accommodation area in motor caravans.
Regulation 46 of the Road Vehicles (Construction and Use) Regulations 1986, as amended, states motor caravans first used on or after 1st April 1982 but before 1 October 1988 shall be equipped with anchorage points for the driver's seat and specified passenger seat (if any); and for motor caravans first used on or after 1st October 1988 shall be equipped with anchorage points for the driver's seat and any forward-facing front seat.

You can download a copy of the SI at http://www.opsi.gov.uk/si/si2001/20011043.htm .
However, this did not preclude manufacturers fitting seat belts to forward facing or rearward facing seats within the accommodation area if they wished to do so.
Where seat belts are fitted they must be worn.

Seats in the rear of a campervan/motorhome did not, prior to October 2007, require seatbelts (whether forward, rearward or sideways facing) and it is not illegal to carry unrestrained passengers in them while travelling, providing the vehicle is not overloaded. It is not something we would recommend, however.

Although current seat belt wearing regulations do not prohibit carrying more passengers in vehicles than there are seat belts available, the police may prosecute drivers for carrying passengers in a manner that may injure someone. [see legislation extracts below]
We would advise that no-one should be carried in any unbelted seat in the rear of a motorhome.

An EU Directive (2005/40/EC) on the installation of seat belts required that from 20 October 2007 new vehicles have to have seat belts fitted on all seats except those seats intended solely for use when the vehicle is stationary.

The original advice given to us by the DfT was that, where seat belts are fitted, from May 2009, the seat belt wearing Directive would prevent more passengers being carried than there are seat belts in the rear of vehicles. This would have meant that from May 2009, in any vehicle of whatever age, where seat belts are fitted in the rear, more passengers may not be carried in the rear than there are seat belts available.

They now tell us:

"There will be no change in the regulations in May - our lawyers tell us that is not necessary because the existing regulations already adequately deal with the requirements of the seat belt wearing Directive. Our earlier view that we would need to change the regulations was mistaken."

"Seat belt wearing regulations cannot apply in seats where seat belts are not fitted. Therefore there can be no specific prohibition on using such seats even if other seats in the rear of the vehicle have seat belts fitted. Specifically, the regulations exempt passengers from using a seat belt if a seat belt is not "available". If all the seats with belts are already occupied, then seat belts are clearly not "available" and the remaining passengers can use the seats without belts."

The advisability of carrying unrestrained passengers is another matter, to quote the DfT spokesman:

'... the police can already act where people in the rear of any vehicle are considered to be carried in a dangerous manner because they are unrestrained. [Owners] should beware of unbelted passengers. In a crash, they can injure others in the vehicle ...'.

In addition to considering the legal and safety issues involved, owners who intend to carry passengers in unbelted seats must check with their insurers to confirm that this is acceptable to them.

Our advice is that, regardless of the letter of the law, all passengers should wear seat belts.

We are indebted to Rohan Pohl and Tim Norman from the Department for Transport for their time and patience in answering all our questions on this subject.

Extract of relevant additional legislation relating to the carriage of passengers:

Road Traffic Act 1991 Section 40A:

A person is guilty of an offence if he uses, or causes or permits another to use, a motor vehicle or trailer on a road when—

(a) the condition of the motor vehicle or trailer, or of its accessories or equipment, or

(b) the purpose for which it is used, or

(c) the number of passengers carried by it, or the manner in which they are carried, or

(d) the weight, position or distribution of its load, or the manner in which it is secured,

is such that the use of the motor vehicle or trailer involves a danger of injury to any person.

The Road Vehicles (Construction and Use) Regulations 1986

(1) A motor vehicle, every trailer drawn thereby and all parts and accessories of such vehicle and trailer shall at all times be in such condition, and the number of passengers carried by such vehicle or trailer, the manner in which any passengers are carried in or on such vehicle or trailer, and the weight, distribution, packing and adjustment of the load of such vehicle or trailer shall at all times be such, that no danger is caused or is likely to be caused to any person in or on the vehicle or trailer or on a road.