As the title suggested, I'm looking to get a sort of sealer to seal the window frame around my Blockwork. What's your recommendation on this? And before I apply the sealer on, do I need to scrape off the old sealer around the window frame?
I'd recommend you use Sikaflex 11FC, which is a flexible polyurethane filler designed for applications like this. It appears the old sealant has shrunk, providing adequate room for the new sealant, so I don't believe removing the old sealant is necessary. Of course, you could do so if any loose sections are easily removable.
I can't quite tell, but from the image, it looks like quite a large gap. You can prevent excess use of the filler by first packing the gap with filler rod. Push the rod into the gap so there is a 10mm gap between the rod and the front of the window frame. Fill this gap with the Sikaflex and smooth it off with a finger dipped in soapy water so the coved filler reaches from the blockwork to the window frame.