Briggs also makes a shielded plug-end boot for the plug wire. However both R-series spark plug and shielded boot make a small difference, unless you are trying to listen to AM radio in peak hour traffic while stopped in an underpass. If you have major-league RFI in the TV band there is something wrong with your ignition system and it will not be helped noticeably by changing the plug or plug boot.
The problems with plastic fuel tanks are mostly associated with using fuel with ethanol in it. Unless someone has deliberately bought fuel containing ethanol, Australian fuel does not have the problem. However the stuff with ethanol is a bit cheaper so it may have been used in your mower at some time, and you should inspect the tank carefully then refrain from using any fuel containing ethanol in future. By all means call the consumer affairs number and pay attention to their advice. There are plastic formulations that are especially susceptible to ethanol damage: I seem to recall that typical glass-fibre-and-resin tanks are vulnerable, but you should check that in case my memory is faulty.
