Have you tried phoning BC direct?
01274 683 633
http://www.bc-racing.co.uk/coilovers
There is no indication on the spec pages that any BC shock is rebuildable. They don't list stuff like seals, rods, valves on accessories page. They may be welded like OEM shocks. Inspection of the damper body seal holder is required. If it has a hex for spanner or holes for a peg spanner it should be possible to disassemble it. If the seal holder is welded to body and the other end is too then it's probably not serviceable.