Jim Crosskey, many thanks for your reply. I have two 11ft Shimano feeder rods which are soft enough for roach but have enough backbone to cast a 2oz feeder, they have carbon tips which I don't fancy altering (yet?!).
Seth 49, many thanks also for your reply, I have a couple of other ideas to try first but I will test the Gardner marker - I didn't know that it even existed.
I went roach fishing again yesterday on a club lake to test out ideas for line markers when quivertipping. The fish are at long last starting to feed, no roach but I did catch six bream which were around 5lbs each. For line markers I tried both 6lb powergum and no. 4 pole elastic, both worked OK but if I tied the knot tight enough to stop the knot from sliding it was difficult to remove. Next I am going to try some white quick dry nail varnish and if that is no good I'll try the Nash and Gardner stuff.
Many thanks again for all your suggestions.