Today my dad came round and helped me to clean and re-grease my rear hub gears. Now the bike is running a lot better. Almost as though it’s now.
However I still need to replace the cogs and chain some time soon, before they wear out completely. As I have internal gears, I can get away with wearing away the cogs for a lot longer compared to other external derailleur gears.
Hopefully I won’t need to take it apart for the next 6 months to a year to do another re-grease.