The indices use the repeat sales method for index calculation. (indexologyblog.com)
The FTSE, or the FTSE Group, is a company, similar to Standard & Poor's, that specializes in index calculation. (investopedia.com)
Update: I raised this query on TMF UK's Index Tracker forum, which provided a crucial insight: it's possible for the total market cap to vary without the index value changing, either due to company share issues / buy - backs, or companies entering / leaving the index (much more detail in FTSE index calculation guide). (money.stackexchange.com)