Very Slow Battery charging

Hi, I have an Apple Watch 4 and recently, after no discernible change in use, it now takes a very long time to achieve any charge at all. I have left it charging overnight and found it is still only just over 1/4 charged. It also discharges incredibly quickly, the last time it was fully charged it discharged to zero in under three hours. I've tried unpairing it to then relink and restart the watch - but now need the phone to get to 50% before I can do anything - I feel stuck! Any recommendations for what I might do?