Ideal classic boiler - gurgling sound before fire up?

Hi hope you can help, there seems to be a few questions and answers surounding airlocks on this boiler although im not expreriencing problems with the boiler cutting out, only this annoying sound.

i recently had a rad valve changed downstairs by a plumber that led to the bleeding of the rads throughout the house. ever since there has been the sound of water sloshing through the pipes into the boiler when it turns on just before it fires up.

Im guessing theres air in or around the boiler although im not entirely sure how to bleed it.

i have tried the rads once more in the hope air had settled in them over the weeks since the last bleed although only a small amount in one rad was found.

i also bled the valve by the hot water tank upstairs which again only gave a small amount of air.

any ideas? there seems to be a small bleed nipple on top of the boiler very much like you find on a car brake caliper, i cautiously turned that earlier without anything coming out at all?

i also noticed a valve on one of the copper pipes above the boiler? i havent tried this as of yet as my luck would have it, i dont know what its for and the world would most probably end

thanks in advance richie.