Cut the nails off.  

The correct answer would be they shouldn't, but that depends why they put the nails there in the first place, it certainly isn't normal practice.

You do get copper tingles or rivets to stop the tiles rattling in the wind so may be a variation of that.

Hi, good idea about the wood, but on close inspection it seems that every other slate is a quite narrow slate with only one nail (through the facia), so I have only 6 slates to worry about.
I have looked at Scruffs 'tingles' and also further searches of internet showed me details of these. So a friend has given me some 2mm stainless welding wire and I have cut and bent to a 'Z' and managed to fix a nail in a loop at one end, under slate and bent up and cut other end, as the copper ones. Have done already two each for the six slates, these are now firmly in position, for good I hope.
Thank you both for the quick replies they really did help, as I think my fix is better than the original.
Thanks again
Trevor