There was some discussion
a while back about using plastic grommets or "hole plugs" from various suppliers to do the same job as the more expensive frame plugs.
A colleague at work was popping out to Maplin at lunchtime so I went with him to buy some of their plastic "hole plugs" to see if they fit.
From reading the website, it appears you buy them at 99p each in a minimum quantity of 10, but on arriving at their shop it turns out it's a
pack of ten for 99p!!
Will post more tonight when I've found out if they fit!