My original battery started swelling up when I was abroad on holiday in the winter (having left when it was still permitted). As something needed to be done, I ordered a battery off Lazada and fitted it myself.
Despite an LG logo, I suspect this battery was a Chinese copy and the performance was inferior to the original.
After returning home, I contacted LG UK asking where can I purchase an original LG battery. They do not sell batteries, but did offer a repair service for £46, including courier transport both ways.
I accepted this option and the battery performance is now back to where it used to be.
Whilst they had the phone, they called me and asked if I wanted a new rear cover for £76, since there was minor damage to the paint from my first repair. I declined, as the phone lives in a protective cover and I cannot even see the rear of the phone.
Also, I now have an app which warns me so I can take the phone off charge at ~85%, which ought to prolong battery life in future.
Conclusions:
#1 Fitting the battery myself was easy and it is a good option if you can accept a third party battery might lose a few % compared with a new battery.
#2 LG UK offered a reasonable battery replacement service, if we consider the cost of battery, labour & transport. £46 is good value if I get at least another year out of the phone.