Here's a part guide to R10's or 'Holy Grails' as they are referred to on Atarimania.
Holy Grails are defined as 3 or less cartridges - whereas R10's can be much higher numbers... take Air Raid for example.
I say 'part guide' because there are a hell of a lot more PAL 'Holy Grails' not listed as such on atarimania, probably because it is a mammoth task grading PAL due to the much larger international scope of production and distribution.
I think of AA for best atari forum on the net, and AM for most comprehensive database on the net.
The best knowledge of PAL rarity comes with time, and there are plenty of wise old PALists here, so by posting a question in these forums as you have done is the best way to find out.