Thanks for the link. I have a fair idea what I need to do now but it's
raised some more questions.

How long is this likely to take? Given that the roof is about 3
meters will I get it done in one day (7 hours)?

Do I need to buy a dresser and/or bossing stick or can I get buy with
a mallet and a bit of wood?

Should I use clips to hold the flashing down on to the tiles (as
described in the wickes leaflet)?

Should I use sealant instead of cement for the pointing (as described
in the leaflet)? I don't think I've ever seen flashing which doesn't
use cement but if sealant works better I'm happy to use it. Sometimes
the new alternatives can be worse though (like the stickey tape which
started this thread).