Since lockdown I have had stiff joints. I haven’t been doing as much running and wondered if this could be the cause?

Stiffness could be down to various factors including, as you mention, a change in your exercise routine. Also, as we are all spending more time at home at the moment, I think we are tending to sit around more than we normally would. So, you may find your stiffness eases with some regular stretching and movement. Take a look at my blog on exercises to do at home for some tips on how you can keep moving more!