[0018] FIG. 3 shows the lock unit 1 and fastener tab 2 . The lock unit has a flat base 3 with a raised wall 4 which defines a seat for the fastener tab 2 . The base has a series of apertures 5 which receive rivets which fix the lock unit on the material of the bag or envelope. Within the wall of the lock unit 1 there is a raised enclosure or chamber 6 which has at one side (the lower side as seen in FIG. 3 ) an opening 7 . The base has a depression 8 which communicates with the chamber, to allow the broken part of the seal to be shaken out of the chamber in known fashion.

[0019] The fastener tab 2 , in this embodiment, has three apertures 9 , 10 , 11 . Aperture 9 is for connection with the zip puller, in known fashion; aperture 10 has a shape which corresponds to that of the chamber 6 , to allow the fastener tab to locate over the chamber; the aperture 11 locates on a raised catch 12 on the base part of the lock unit, again in a known fashion.

[0020] The important and novel feature of the lock unit and chamber seen in FIG. 3 is the presence of the shoulder 17 at either side of the opening to the chamber (the shape of the shoulders is also seen in the later sectional FIG. 7 ).

[0021] FIG. 4 shows the disposable and frangible seal 13 which has a head part 14 and body part 15 . The head part has an arrowhead configuration with barbs 16 , in a known fashion. The body part is, in this embodiment, substantially square with rounded corners, but of course it could be of any other desired shape. Between the body part and head part is an elongate recess 18 (see most clearly in the bottom view) which provides for the weakened and breakable connection between the head and body parts. Also, on the underside of the seal are recesses 19 in a position corresponding with the shoulders 17 of the chamber. The seal also has a central recessed portion 20 , which is entirely optional, but reduces the use of material in the seal. In a less desirable embodiment the material between recesses 19 could be omitted to create a single, wide recess.

[0022] A further feature of the seal is the indent 14 a in the head part 14 .

[0023] As shown in FIG. 5 , to close the bag or envelope the fastener tab 2 is located on the chamber 6 of the lock unit 1 in a known fashion. This position is shown in FIG. 5 . It can be noted in particular that the aperture 10 of the fastener tab registers with the chamber, including the shoulders 17 , while the aperture 11 locates on the raised catch 12 of the base of the locking unit. The height of the shoulders 17 is greater than the thickness of the tab 2 , so that they protrude above the tab.

[0024] To finally lock the envelope in a tamper evident fashion, the seal 13 shown in FIG. 4 is pushed into the chamber 6 , the head part 14 of the seal locating within the chamber and in particular the barbs 16 locating behind the chamber walls in a known fashion, so that the seal cannot be removed without breaking the body part 15 from the head part 14 . In this position, the body 15 of the seal overlaps part of the tab 2 . The locked condition of the envelope is shown in FIG. 6 , those parts of the seal and chamber which are not immediately visible being shown in dashed lines. FIG. 6 in particular shows the location of the shoulders 17 within the recesses 19 of the body part of the seal and the location of the indent 14 a at the end of the head part of the seal on an inner protuberance or protrusion 21 on the inside of the chamber.

[0025] The sectional view of FIG. 7 shows the seal 13 approaching the chamber 6 ( FIG. 7 a ) and then the seal located within the chamber ( FIG. 7 b ). The views show in particular the shoulders 17 and the protuberance 21 within the chamber. It can be noted that the opening 7 of the chamber is tapered, that is the edge of the wall of the chamber above the opening is inclined with respect to the plane of the base 3 of the lock unit. This feature allows a close tolerance to be achieved between the thickness of the seal 13 and the top of the opening 7 , while at the same time the tapered shape provides no hinderance to the seal being inserted.

[0026] It will be appreciated that by the location of the shoulders 17 in the recesses 19 of the seal, any significant lateral movement of the seal is prevented while at the same time any significant downward movement of the seal is also inhibited. Accordingly, the risk of the seal being manoeuvred to a position relative to the chamber where a gap is left into the opening is reduced.

[0027] To open the envelope, the seal is broken and removed in a conventional fashion (this removal providing tamper evidence of an authorised opening). The head part 14 of the seal is shaken out from the lower part of the chamber which communicates with the depression 8 in the base 3 of the lock unit. This depression extends the width of the chamber, beneath the shoulders, so that the width of the depression is greater than the width of the opening 7 which initially receives the head of the seal.

[0028] The lock unit may be formed of a suitable material, such as hardened steel, or of plastics such as nylon, polypropylene, ABS, styrene or engineering plastics. The disposable seals, which will generally be supplied in bulk, are formed of a suitable resilient but frangible plastics material, such as styrene, nylon or polypropylene.

[0029] Although the invention has been described with reference to a zipped envelope it is equally relevant to other types of containers. For example, the container could have a flap with an aperture which registers with a lock unit on the main part of the container, the seal then going into the lock unit to secure it.