[chris (OP)]

Why can some plastics - like ready-meal dishes - be cooked in the oven whilst others go soft even when hot water is poured on them? How is this achieved chemically?

[Chemistry4me]

Thermoplastics can be remelted and remoulded, the chains are held together by weak van der Waals and/or hydrogen bonding, this gives some of them their low melting point.
Thermosetting polymers (e.g Bakelite) are cured so instead of them being a chain, a cross-linked polymer can be obtained by controlling the stoichiometry of the reaction. This cross-linking gives a larger molecular weight, giving the material a higher melting point. A thermoset material cannot be melted and re-shaped after it is cured. It'll decompose before it melts.