A shelf came down due to the plaster around the holes wearing away, and heavy things being put on the shelf, so I wrapped some old bike inner tube around the rawl plugs, and twisted them into the holes, before tapping them fully home with a hammer, and cutting off any excess rubber after screwing in a screw into each one and pulling with some molegrips to test the hold.
I now have a sturdy shelf, and the problem was solved for free. Hooray.
Post edited at 14:41