Hi Andrew,
First up, this has nothing to do with copyright. The relevant law is that of design right, and in particular, registered design right, the details of which are contained in the
Registered Design Act 1949, which has been amended by quite a bit of subsequent legislation.
However the relevant part for the purpose of your question is section 7A(5) which says:
(5) The right in a registered design of a component part which may be used for the purpose of the repair of a complex product so as to restore its original appearance is not infringed by the use for that purpose of any design protected by the registration.
This provision was added to the original legislation by the Registered Designs Regulations 2001 which were enacted to reflect EU law, and are mainly intended to cover the after-market for things such as car parts.
The effect of section 7A(5) is to permit the sale and fitting of parts which look the same as the original parts, but which are not made or supplied by the original manufacturer. In other words, exactly what you propose. The material from which the replacement parts are made is not relevant as far as design right is concerned.
However the chioce of materials may be relevant under the
Road Vehicles (Construction and Use) Regulations 1986 as amended, where safety is concerned, for example with regard to crumple zones and pedestrian safety etc. Obviously that is way outside the subject matter of this forum so you would need to do your own research on that.
