Rent increase twice in 3 months?

[KelleStar]

DSis is renting a 2 bed mid terrace cottage in a rural area. Her rent went up at the end of November by £80, she's just had another letter from LL with a £170 increase. Since she's been there, 3 years, it's gone from £500 to £860. BiL doesn't want to leave the area as his parents are close, but no other properties for rent locally.

Personally, I think they no longer want to rent out the property and move in as owners, and are trying to up her rent so they clear off. LL bought the house 6 months after DSis moved in.

It's not worth it, poor heating (oil) front door is gappy and let's draughts in, windows are lead/glass panels that are wobbly and also let draughts in.

Is there a loophole anyone can think of as they'd prefer to stay.

We are relatively new LL so maybe not up to date etc but we were under the impression the rent could only be increased at the end of the contract and at no more than inflation ie about 3%.
We have just come to the end of the first year with one of our tenants and have increased it from £795pcm to £810pcm
We do not intend to increase for another year.
Did your DSis rent through an agency or privately?
She should maybe speak to an agent about the terms and then put it to the LL that it is unreasonable?

It's an estate rental, all properties on the area are leasehold or rental, it's a rolling rental not due until July... She's trying to tempt her DH with a bigger property nearby on the outskirts of a town for less money.

That jump is too large for it to be legal.

3% or rate of inflation, annually, whichever I less, I think the rules are.

I've told her to complain, or at least ask them to make some repairs that have been promised for years.

there is no legal limit. It's not worth it, so she leaves.