Radiators - where to buy?

[Butchx5]

We are going to extend our existing central heating system and are looking for the radiators we need - without much success!

The largest we can find is double-skinned, 600mm high by 1200mm long - admittedly the wattage of heat output is quite good - but this is much smaller than the radiators in the rest of the house and our rooms are all quite large (36m² x 2.5m high). It would make life very complicated to fit two or more rads per room - before this is suggested

Does anyone know of any outlets where we can purchase/order larger radiators - eg double-skinned, 1000mm high x 1200mm long?

Thanks

Mike & Helen