I took note of what a few people said on here but also searched about what the cause could be and what to do on other sites too.
I'm glad to say that Sunshine's fins have started to regrow (almost fully now, except the colouring hasn't properly matured yet, its still slightly translucent)!
This is what I did:
1. I cleaned his glass gems, his Greek arch, rinsed the java fern, wiped the tank after filling it several times with really hot water, and then did 100% water change
2. I removed the filter
3. I turned the temperature up to 30 degrees celcius
4. I ordered some Melafix (I know some people urge others not to use it but I was desperate!) online and it arrived a few days later, Sunshine's fins hadn't improved but hadn't got worse either by the time it arrived
5. Every day I put about 1ml of Melafix into his water (I did this for 5 days)
After 2-3 days I noticed that some of his fins had started to grow back and that the tears were healing. The redness around the tears of the fins had also gone.
He also became a lot more active and flared a lot more often (before I used Melafix on him he was very placid)
After 5 days (today!) his tears had almost fully healed and likewise his fins had almost fully regrown so I decided that Melafix had done its job and stopped using it. I did a 100% water change (as the Melafix had caused a translucent scum on the surface) and it was due its weekly clean anyway. He seems fine now.
Theories regarding what caused the fin damage:
I'm a betta novice, but I don't think it was fin rot. I clean the tank weekly and have 100% water changes, I ensure the water temperature is at least 25 degrees celsius (in fact, its almost always 27 - 29). I use water conditioner when I do the water change and I also clean his glass gems and his arch in hot water.
My theory is that my betta had noticed a little flag I have near his tank. The flag has a shiny golden 'ball' at the end of the 'stick' that it is on, and the 'ball' does have a mirror effect. I believe that eventually he had started to notice this 'ball' and it had frightened him. I tested this theory, I put the 'ball' up close to his tank as soon as him he flared immediately and showed his fins off and followed the 'ball' as I moved it round in front of him. I'd never seen him act so aggressively before. So I think that he'd seen the golden 'ball' and possibly seen his yellow tail from turning or after seeing his reflection and thought that his tail and the ball was another betta and as a result started biting his tail.
Since this discovery I've moved the flag and like I say he has almost fully recovered. I know this theory probably seems unlikely but I believe it was the cause.
I occasionally do show him the 'ball' so he can flare once in a while but it really does get him going!
About the filter. The filter didn't really cause the water to move round much and I don't believe it was pushing him to swim against a current. However, as the water fell from the filter back into the water it cause some sizeable bubbles to fall deep into the water and back up although I didn't see him go near them very often.