For New WEEE (sold after 13 Aug 2005) The producer will be responsible for the costs of collection, treatment, recycling and recovery for all WEEE they have put on the market unless they have contractually passed the obligation down the supply chain.

For Historical WEEE (sold before 13 Aug 2005)
If no like for like replacement, the end business user is responsible for disposal
If like for like replacement of equipment is purchased,  the supplier (producer) is responsible for safe compliant disposal

Advantage Waste Brokers Compliance Scheme is an open Producer Compliance Scheme and provides a choice of services to businesses obligated under the new Regulations.

 A brief summary of business users obligations

http://www.defra.gov.uk/environment/waste/topics/electrical/index.htm

WEEE business user fact sheet (2 pages) New rules for disposal of WEEE

Guidance notes for Distributers / Producers of EEE (6 pages, page 2 describes B2B)