There has been a bit of a history of problems with the windows. The guarantee on these passed some time ago and thus responsibility for repair / up keep does fall to the leaseholder. That said if the leak is due to the window interface with the rest of the building then it may be a different matter. It really would be best to contact RMG with regards to arranging repairs as they have contacts with the supplier of the original unit.
Our property manager is away on holiday at the moment. I would suggest emailing RMG with details of the problem and they should pass on the contractors details and advice.
Kim, your problem would have to be addressed by your landlord. If he is unwilling to do it then I would make sure he signs off that he cannot make a call on any deposit for said damage when you move out.
Hope that all helps.
All the best