3. I am not the "registered owner" (my name is not written on the title deed of the flat which has been registered in the Land Registry) but I have paid part or all of the purchase money of the flat. Do I have a say in matters concerning the property? For example, can I prevent the "registered owner" from selling the property?
Although you are not the "registered owner", if whole or part of the purchase money comes from you, you may be the "beneficial owner" of the property. That is to say, the property may be held by the registered owner in trust either wholly or partly on your behalf. If you and the registered owner have respectively contributed half of the purchase money, your half share may be held by the registered owner on your behalf. You should immediately notify any intending purchaser and his solicitor of your beneficial interest. This could effectively stop the sale. Alternatively, you could claim against the registered owner for your share of the proceeds if the property is sold.
However, if you intended to buy the property and give it to the registered owner as a gift (in which you had clearly expressed that when you paid the money), your beneficial interest in relation to that property is arguable. That means you may not be able to stop the sale or claim for your share.
The above matters involve complex legal arguments in which legal advice must be sought.