I guess this is a big day. Carpets going down. :0)
We are using low TOG (0.8) underlay because of the underfloor heating. Bizarely, the underlay is heavy, but it is about the heat transmission quality of the underlay rather than the weight of the material. The carpet is also a low TOG (1.4) carpet from the Cormar Glendale range.
As we have tried to do throughout the project, we are using local suppliers.