The good news is yes, they do fit!
The 30mm fits lovely although is a little loose to start off with, so I bent out the tabs around the edge and it's now wedged in there lovely.
However, the smaller ones are only 19mm across but the hole appears to be about 22mm. The plug does cover the hole but it's very loose, so I used some double-sided foam tape wrapped around the edge to hold it in place, which it does nicely.
The plugs look really good on as they are a matt-black effect plastic which goes great with my grey frame and other black accessories on the bike.
I'll take some pics tonight and put them up for you to judge for yourself.
For those that want some, the order codes from Maplin
are RG80 for the 19mm (small) plugs (99p for a pack of 10) and RG81 for the 30mm (large) plugs, again 99p for a pack of 10. You could try mail order (sign up for their email newsletter to get a 2 quid off any order discount voucher) but going to your nearest store is probably the best idea.
So unfortunately I haven't stumbled across the big money-making plan to free me from working for the rest of my life, but I've just saved myself a few quid to put towards the next project...