plodfather - tell plodmother to look on the gear thread. Loads of threads on sports bras. And will save your blushes.
If she can't find one she likes, apparently DavidB will do a free fitting :-)
Alternatively: www.lessbounce.com (they do a mail order catalogue too, and the advice is v useful, both the written descriptions and over the phone) or other people have recommended www.sportsbras.co.uk
I wouldn't buy one from JJB, the only ones I have ever seen there have been fashion sporty crop top style ones only suitable for pre-pubescents. I am assuming the plodmother is not under 12...
If she just wants to get it over and done with, John Lewis sell a limited range, but it does include ShockAbsorbers. They seem to be the most popular choice in my gym and they're comfortable.
Unfortunately without knowing what was wrong with the bra, can't suggest anything more specific, but I'm sure other people will come up with some imaginative explanations. I'm too tired to try to be funny...