Some of the best things in London are free - museums, parks, the view from the river - but if you want to spend more than a day taking in all the sights, the cost of a school trip starts to rocket. However, despite its reputation as one of the world's most expensive cities, it is possible to stay over and still keep the price reasonable.
Without a doubt, one of the best deals around is the Kip on a Ship sleepover available on HMS Belfast. The former battleship is in a prime location berthed opposite the Tower of London and close to the London Dungeon, Tate Modern, Design Museum, Imperial War Museum London, and the Globe Theatre.
Schools groups stay on the mess decks which have room for 52 pupils (aged eight and over). Adults have their own private cabins close by.
Original fittings from the 1950s remain, including a steel ladder down to the mess decks and three-storey bunks - although only the first two levels are used. Students get their own battered but sturdy locker and a television and video recorder is available for evening entertainment.
Packed lunches are available for pound;5.50. Individual groups have exclusive use of the mess decks and a member of staff is available throughout the night.
You can book one of the many free education sessions run on the ship, or take a tour of the ship's exhibits. All this is available for pound;20 per person per night, but stays are limited to three nights.
Close behind for value is the Icando centre, run by Girlguiding UK.
Situated opposite the Royal Mews, the activity centre caters for groups of up to 40. It is aimed at boys and girls aged 7 to14 and it has separate sleeping areas for children and adults, full disabled facilities and a good selection of in-house entertainment including air hockey, karaoke machine, bridge-building sets and a puppet theatre.
The centre is imaginatively decorated with fake tree trunks hiding lockers, indoor picnic areas and touch-screen displays. Children can sleep on the floor anywhere in the centre. Prices for an overnight stay for a group of 24 costs pound;22.40 per person.
Combine rural retreat and the buzz of the city with a stay at Northern Heights, a 7.5 acre-meadow and ancient woodland site between Elstree and Edgware in north-west London. The site, owned by Middlesex East Guides, includes Fieldside, a large dormitory that can sleep 24 and Blackthorn, a modern cabin with disabled access which has four bedrooms, sleeping a total of 12. Both have kitchen facilities, showers and dininglounge areas.
There is also plenty of camping space: enough room for 200. Other facilities include a shower block and a building for indoor activities. The Earth House, a new 38-bed unit, is due to open in the autumn.
Fees cost pound;85 per 24 hours at Fieldside or Blackthorn and camping costs pound;2.50 (pound;3.50 with supplied equipment) pppn.
The Youth Hostel Association has nine hostels in London - three of which cater specifically for groups: Kensington, Rotherhithe (Docklands) and Hampstead Heath all have kitchen and laundry facilities, a TV room, lockers and luggage stores. Prices start from pound;18.50 for students under 18, and pound;21 for adults.
Other cheap accommodation can be found in university halls of residence.
Most are only available from June to October, but some are available at Easter.
University College London, with sites in Bloomsbury and near Camden, north London, offers self-catering rates starting from pound;19.20 a night.
Prices at City University, near the Barbican, start from pound;19 for groups of 10 or more. Imperial College, London School of Economics, the University of Westminster, the Institute of Education and Brunel University in Uxbridge also offer budget accommodation.
As well as offering affordable accommodation, many sites offer discounts to local attractions.
Find out more
Visit London produces the free booklet, 'London - The Official Guide to Where to Stay on a Budget'. For an information pack call 0870 240 4326; email: email@example.com; www.visitlondon.com.
'HMS Belfast' Tel: 020 7940 63236336; email: firstname.lastname@example.org; www.iwm.org.ukbelfastindex.htm.
Icando Tel: 020 7592 1818; email: email@example.com; www.girlguiding.org.uk.
Northern HeightsTel: 020 8458 5549; email: firstname.lastname@example.org.
YHA Tel: 0870 770 6117; email: email@example.com; www.yha.org.uk.
University College Tel: 020 7679 1479; email: firstname.lastname@example.org
City UniversityTel: 020 7040 8037; email: email@example.com; www.city.ac.ukems.