A diffuser is a metal plate you put on the gas burner to keep the heat lower - see here (http://www.lakeland.co.uk/heat-diffuser/F/keyword/diffuser/product/6280_10426) for an example.

Galangal (link to BBC page (http://www.bbc.co.uk/food/glossary/g.shtml?galangal)) is similar to ginger - it's wonderful fresh, although not easy to get in this country. You could substitute fresh ginger.