Where a mortgage purports to secure a principal sum the amount of which is specified therein (whether or not the mortgage also purports to secure further advances), the mortgagee shall have the right to advance from time to time to the mortgagor the whole or any part of the principal sum the amount of which is so specified so as to rank in priority to any subsequent mortgage, notwithstanding that the advance is made after the execution or registration of the subsequent mortgage, and whether or not the mortgagee has actual or constructive notice of the subsequent mortgage at the time of making the advance:

“Provided that any part of the principal sum which has been repaid to the mortgagee and readvanced to the mortgagor shall be deemed for the purposes of this section not to form part of the principal sum specified in the mortgage:

“Provided also that nothing in this section shall derogate from the provisions of subsection four of section one hundred and two of the Land Transfer Act 1952.”