I think they might be going further down into the soil now
Dave..they will be there somewhere..and the red wrigglers (Brandling worms)are usually found in the compost bin,they don't live or survive outdoors ..mine are not as visible in my bin now..
but come the warmer weather again,there will be masses of them..

most worms will be going down below 6" deep now. there have been light frosts so they will be going down for warmth.
no point adding any brandlings, as previously mentioned they are not a true deep soil 'earthworm' as their natural habitat is in the leaf litter of woodlands. hence loving the compost bins.

I agree with the comments above but the frost has not been freezing the soil in our area yet. Do you live under the flight path to an airport. I ask because when we moved here there were no worms in the garden. At that time we were under the flight path in to Edinburgh Airport. The planes flying over caused a vibration in the soil which made the worms think it was raining and they came to the surface and were snapped up by the birds. The same vibration process is mimicked by seagulls. Have you noticed them doing a sort of war dance where they walk on the spot. They are worm hunting. We poured tons of farmyard manure in to the garden every year. We were buying so much the farmer thought we were growing mushrooms. The flight path has been changed so we rarely have planes coming over now.