I would like a list of the 100 largest companies as quoted on the
London Stock Exchange in 1964 (i.e. forty years ago).  I need the list
of companies and their market capitalisation (i.e. share value) in UK
Pounds.  I would also like to know any sources of such historical data
for future research.  Thank you.

I'd suggest the Stock Exchange Year Book for that year as a starting
point; a commercial library in London might have it for reference.

Owain