So today I spent my time, with my brother in law who knows much more about bikes and working on them than I do, cleaning the bike, wirebrushing the metal parts and spraying it new, and I was telling him about my problem with the gears, and he checked the chain, and said it was waaaay to slack, so he tightneed it up and then I went out and test it, it felt like a different bike, the engine breaking wasn't nearly as severe, I could actually glide a bit while in gear, I liked it alot more. Then while cleaning the nuts and screws, I accidentaly unscrewed the oil cap screw, and lo and behold, a litre of oil fell out, onto the garage floor, jet black. So we went up halfords and got some Castrol Power 10W40 and put that in, and I'm glad to say, everything working perfectly. Feels like a different bike, it even seemed to handle better but I think that's because I feel more confident cornering knowing that it wont slip into neutral mid-corner. Much happier with it