Someone used an old petrol powered leaf blower around my house. i breathed in some trapped fumes which were inside. can this affect my brain?

Inadequate duration: Open the windows and air out your flat. Get out in the fresh air and any carbon monoxide or other fumes inhaled will out-gas and be gone. If severe or frequent fume exposure, consider an carbon monoxide detector for your home (not a bad idea anyway), as co is odorless, colorless, and lethal.

Very unlikely: It is very unlikely that inhaling petrol fumes will cause any ill effect to your brain.

Yes but temporary: Petroleum exhaust fumes contain carbon monoxide and hydrocarbons that are not healthy to breathe in. They can affect the lungs ; cause dizziness/lightheadedness ; headaches. Chemically sensitive patients may experience "brain-fog" (problems with memory and concentration). But the amount you were exposed to is likely minimal and any adverse effects should be mild and temporary.