The one at Marylebone has been supplied by
ape2o, who've also installed them at Basingstoke, Bournemouth, Hampton Court, Kingston, Oxford, Putney, Richmond, Twickenham, Vauxhall and Wimbledon.
In the case of Marylebone, the
Refill app suggests AMT Coffee and Starbucks offer free refills and I believe Greggs may do so as well, though it's not listed.
Though equally, a charge of 25p for up to a litre of chilled still or sparkling water is significantly cheaper than anywhere else within the station.