In the present case the rod 3 is flexible throughout its length (the rod 3 could however be flexible for only part of its length) and may be made of for example an elastomeric or thermoplastic material, selected particularly from the group comprising: a silicone, a fluorosilicone, a butyl rubber or isobutylene/isoprene copolymer, a nitrile rubber or nitrile/butadiene copolymer (NBR), VITON??? (DuP