In the series of Nigellissima, Nigella uses a pan which looks like two saucepans which the small one fits into the larger one. It looks like the old fashion pie tins, white with a blue rim round the edge. Can you supply the suppliers name and address please? I searched the web without any luck. Kind Regards, Mandy.
The pan that you describe is known as a bain marie, or sometimes also as a double boiler. It is made up of a small pan that sits over a larger one. The larger pan contains boiling water and the water heats the contents of the smaller pan. It is a gentle way of cooking and is good for delicate items such as eggs (scrambled or egg custards) and chocolate.
The enamel style of bain marie is a vintage item and it is easiest to find these on eBay, or at car boot sales. They are usually fairly inexpensive. You can also buy new bain maries in many cookware stores, though now the style is for stainless steel rather than enamel. It is also easy to set up a bain marie at home by putting a heatproof bowl over a pan of boiling water.
If you are cooking eggs and custards then the water can touch the bottom of the bowl. But if you are melting chocolate make sure that the water does not touch the bowl as otherwise the bowl will become too hot and the chocolate could scorch.