Hi - I actually phoned the Sonos support helpline a day or so ago and they helped me out.
A little more information in case it is helpful for others. The crackling sound was only when watching TV and was present no matter what the source (DVD, TV box, Chromecast, or smart TV app). It started fairly randomly - I wasn't changing the volume or anything - and persisted when the TV was turned off and on again. On the phone to Songs support, I unplugged everything one by one, restarted the Beam (i.e. unplugged it. Waited a minute or two and plugged it back in) and then plugged everything back in one-by-one and it sorted it out. Apparently, Chromecast can often interfere with the connection somehow and cause the crackling. So far it the sound has been fine since so hopefully it is the end of it!
Thanks for getting back to me though!