section 21 or another

Possession - Section 8 and Section 21

good afternoon, i have a tenant, she is subletting and has been for 4 yrs, i am also charging £600 a month less than the rate I've been advised from estate agents, I have tried many times to upgrade the bathroom nd kitchen but as he has 2 other people living there i cannot do this, she is also complaining about 1-2c degree variation in the heating, i have had a new boiler fitted against recommendation from the plumber as he said there wasn't a problem, now I'm constantly changing thermostats to try and keep her happy, she also doesn't open the windows and it has caused mould, she doesn't clean, she has a dog, cat and fish, I allowed the cat, but not the rest, I just want to take back the house , renew the kitchen nd bathroom and rewnt out at the right amount, she's had 4 yrs at that reduced amount, but now my tax is so much I have to rent for more. her contract run out in last month. do i give her a section 21.?