The company which has been used by RMG is
Metal Window Renovations
27 Tentelow Lane
Tel: 020 8813 8200 / Fax: 020 8843 0836
This is not necessarily a recommendation and there are many companies who would be able to do it.
Part 1 of the 3rd schedule of the lease sets out that the glass and frames of the windows are demised to the lessees.
Hope this is helpful.