I never noticed it at the time, but the AAIB's report into this incident was published in June 2010 and is available here:
Air Accidents Investigation: Boeing 747-436, G-CIVB
I was surprised to read that:
A detailed investigation was carried out over a four day period by the operator, in conjunction with the aircraft manufacturer, and no source of the fumes/smoke could be found. The aircraft was ferried back to the operator’s main base where further examination and testing was carried out, but still no source or explanation of the fumes/smoke was found.
I would have thought that the seats where the sparks/smoke was coming from would have been a good place to look for an explanation. Apparently they couldn't find anything though and the aircraft returned to service 10 days after the incident.
The cockpit voice recordings were accidentally recorded over during the subsequent maintenance activities so were not available to the investigation team. The only reccomendations from the report are that BA and the CAA review their procedures and training regarding CVR preservation.