Thanks @Ed-Pete and @Penguine68,    My neighbour has a combi but still struggles with the pressure.  An independent plumber, not Brit Gas, fitted it.  Brit Gas said one might work in my flat but they would not fit it.  I guess they do not want to be made liable for due to a veritable outside of their control.    I will give Thames Water a call.  I suspect the feed into the house meets their minimum service requirements so I will need to get the work done myself (by someone like Dyno).