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).