Knauf is the name of a manufacturer, earthwool is possibly an unorthodox term for mineral wool which is actually what used to be called rockwool and is just that, made from molten stone. Celotex on the other hand is essentially rigid foam in sheets/boards. All insulation is worth having, not least if you have little or none to start with. There are an awful lot of "shark"-like operators in the area of insulation, etc. arising from grants on offer through government incentives aiming to improve energy efficiency and reduce the carbon footprint. The people who contacted you may be reasonably respectable but you need to be certain that the work they propose to do would not be done as well or better by someone else and also for free. The "free" offer may be in some respect limited and/or not make the best use for you of the grant they intend to take advantage of. Perhaps others can advise you in more detail.