This firm hires 3 berth Motorhomes for about £440 a week kendall. The VAT refered to is at 17.5% and the price quoted on the web site is from 3 years ago but think it should still work out not much more than £500-£550 a week. Obviously an email to them to check current rates is advisable. http://www.agm-apollo.co.uk/

This company also has simmilar rates for 2-4 berths
http://www.elite-motorhomes.co.uk/rates.htm

This company seem to offer a lot of Caravan for £550 a week.
http://www.gm-motorhomes.co.uk/fourberth.htm

as do these people at £520
http://www.home-n-away.com/

This company also look like they may be worth an email and they are close to Heathrow. http://www.leighway.fsnet.co.uk/

This company are more expensive but give a second week free
http://www.rdh.co.uk/tariffs.htm

Others worth a look are
http://www.vivanti.co.uk/holidays.htm
http://www.westcountry-motorhomes.co.uk/rental_v1.htm

Hope this gives you a few more options.