Hi @Lele101 what a shocking late night text to be receiving from your Landlord, that's so unprofessional! I'm a LL myself, and that increase is shocking and astronomical. The saying is "little and often", when it comes to LL's discussing rent increases, so say up by £50/mth every 12mths for example. I'm one of the few who rarely increases rents for existing tenants.
I've just taken a look on rightmove, and it depends on the condition of your flat, but the lowest on there is on at £1500, most around the £1600 - £1800 mark, the brand new ones are on for stupid money £2K+. Whilst the LL can raise it by whatever they like (within keeping of any specific rent increase terms in the AST), you can absolutely refuse or negotiate, or take it Tribunal.
So, now, you should reply to your LL and tell them that you cannot afford that increase and either leave it at that or offer a counter increase say to £1500. If you do not accept the increase, then your LL will likely send you a Section 13 notice of rent increase, giving at least one full month's notice. Once you receive that, you have a small amount of time to take it to Tribunal. At Tribunal, they will look at market rents, and decide if the increase is reasonable or not. They will decide if the proposed increase is reasonable and stays, which then you must pay, or if it's too high (looking at current market rents), then they will set a lower rent for you to pay instead. There is the risk that the Tribunal deems that the increase is too low and may then set an even higher rent for you to pay.
So those are your options.
In my personal opinion, as a LL, I'm rather appalled at LL's who just decide to pass on their huge mortgage increase straight onto their tenants. Being a landlord is running a business, providing a service. It's a risky business when we have BTL mortgages and do not own the properties outright. It's a risk we choose to take, and I don't think it's fair that some LL's refuse to absorb any of these huge interest rate hikes themselves. Without wanting to give you my life story, my first rental was my own home, due to having to relocate, the house was in negative equity at the time due to buying in a peak and then experiencing a decline, so had no choice but to let it out. My mortgage was significantly higher than the market rent, then I had maintenance/repairs and insurance to pay on top. So for the first 3 years of being a LL, I was making a loss, but you know what, that's the property game, that was my problem, it wasn't my tenant's fault that I bought in a peak and had high mortgage repayments! I've since gone on to buy BTL's and have long-term tenants in all of them and don't give hideous rent increases like what you've had, because, I always think to myself, if I got a text like you've just had, and suddenly had to find another £300/MONTH, I'd probably have a heart attack. There's NO WAY I would put that on someone. Your LL needs to learn that they will make less money when the interest rates are higher, and shame on them for just passing the full increase onto you.
Sorry for the huge comment, I just get so irked when LL's do these massive rent increases, it's so awful.