My thanks to everyone for their advice.

I have tried both the old coathanger and the marker on a rigid strip.

The floor is now down but I am NOT very pleased with the result, my
cutting was not very good and I broke 2 jigsaw blades in the process!

The gap between the plinth and the flooring board is not huge and in
places actually touches the pedestal but it is too much trouble in a
confined space to take it out and replace more than a few times.

I have used silicone from Dow Corning to seal the floor joints and it
looks ok.

Thankyou again for your ideas, I really appreciate the replies,
Regards, Peter.

PS The best of the season to you all, no matter your persuasions.