<p>A cap has a domed upper surface (17), a tubular stem section (18) and braces (19) extending from an underside (20) of the dome on to the tubular section (18). Moulded into the dome is a barrier plate (23) which in this case is a thin metal circular disc about 2mm thick. The plate (23) has an annular flange (24) and a central depression (25). The annular flange (24) is concave downwardly providing an internal metal barrier which is curved around its marginal edge forming the flange (24) and generally follows the curve of the dome.</p>
