Thanks for your reply.
As regards XXXXX XXXXX the rent on your Flat itself, your Freeholder certainly can not put up your rent for the hell of it. You will have a Lease in place which will stipulate what the yearly Lease is. Your Lease may say that the rent will increase every 10 years or so, and if it does, then of course the freeholder can charge the increased rent at the due times. However, subject to this, you and your Freeholder are "stuck" with whatever the Lease says the rent will be.
Moving on to the garage, as this is within the same Lease as the neighbour's Flat itself, it is normal for there to be a clause in that Lease to say that the Leasholder (your neighbour) is not permitted to sale part only of his property (ie sell only the garage, as opposed to the garage and Flat). I am therefore guessing that such a clause exists here. If so, I'm afraid the Freeholder can either refuse consent for your neighbour to sell the garage to you, or he can agree and stipulate what conditions/payments are made.
If there is no such clause (which would be rare to be honest), then your neighbour is free to sell you the garage, and the rent that you will pay will have to be apportioned between the neighbour's flat and the garage (I note the toal rent at the moment is £50, so it would probably make sense for you to pay £25 for the garage and your neighbour also pay £25 for his flat).
I hope this answers your question, and if so, I would be grateful if you could leave positive feedback.