Fine, having discovered that a programme exists, how do I contact the owners of this particular archive?
There is a list of television archives on the excellent Kaleidoscope website at address http://www.kaleidoscope.org.uk/tvcomp.htm Please be advised that these companies do not have the resources to reply to casual enquiries from private individuals and certainly not to go on fishing expeditions to see if they might have a programme on a particular topic. A cogently written request giving a known programme title and transmission date may bear fruit but the BBC and some other television companies sell copies to the public only if, say, that person or a close relative actually appeared in the programme and then at a substantial price. Some of the ITV regions sell VHS copies of programmes in their archive to enquirers: certainly Pearson (owners of the Thames back catalogue), Granada, Carlton/Central do… prices range from £75 to £265 per show, then VAT, depending on the region.
