I became aware of ThermoneX at the Build-It show in Birmingham. The following, taken from their web site, sums up why I opted for their solution.

ReadyMade Basements Limited now own Thermonex

A ThermoneX basement however is an all-in one solution, and it can be installed in just a matter of days. Developed in Sweden to exacting design standards, it provides all of the following in one precast element :-

Their MD (David Aulton) has been extremely helpful throughout the design & procurement process which included a meeting on site with my architect. You can contact them via the web site or via:
ThermoneX (UK) Ltd, Delcon House, 65 Manchester Road, Bolton, Lancashire BL2 1ES
Tel: 01204 559551. Fax: 01204 559552