This site claims to offer a Buy-Back deal , and although it says 2 years on the website , Im sure he told me 5 years on the Phone the other day.
( I say claims because they need to be in Business in 2-5 years time for your to return )
http://thecontainerman.com/
I am also looking for one, there is not much difference price wise between a 20ft and 40ft container.
Tengu , if your looking down in the Devon area ( I think your still training down here at st. Lukes? ) Heaver Bros do some for sale or rent at Exeter Airport.
Purchase wise , As i priced it up in the last month
20ft = £800-£1000 (USED STATUS )
40ft = £1000-1200 (USED STATUS ) 40FT =£1700-£2000 ( "GRADE A1" )
Rental , most places seem to charge £100 per month.
Delivery Cost seemed to be £200-£300 .