It looks like the beginnings of a bagworm infestation. If so, those clumps of dead twigs should contain a caterpillar, during the day. At night, they come out to feed.

Be sure the roots aren't too wet & perhaps a more exposed location would help it along. It seems very sheltered. Wind, snow & sun will get rid of those suckers.

Actually it's quite exposed to wind (no shortage of that in wellington) and we've had plenty of sun this summer. It sounds like if I keep an eye on cutting out all those matted areas I'll get rid of the caterpillars. Thanks for the id.

Hopefully, but its unlikely the trees will fill in the gaps again.

The bag worms have now spread to some other thujas I have. Does anyone have any suggestions as to the best spray to get rid of them. I have tried picking off all their cocoons but they keep coming back (possibly from other trees on neighbouring properties)

Here in the States, I would recommend spinosad or Bacillus thuringiensis kurstakii, but I don't know what is legal in the UK.

I heard a lot of positive things about spinosad. I hope I never have to use it.

Thanks for that. I'll see if I can get it and will give it a try (I'm in New Zealand)

Oops! Sorry, Rhys! Somehow managed to read it wrong, but I don't know how. Maybe it's time for new glasses. 8)