Temp warning light

Hi I've been riding my t bird 96 on a p plate with no issues but only short runs I looked at the coolant level and it was down the bottom of the expansion tank so I purchased some triumph coolant took the rad lid off along with the expansion tank lid filled the expansion up to the level then topped the rad up I buttoned everything up went for a ride after 3 miles temp light came on stopped the bike head was to hot to touch but the rad was cold could it be an air lock ? Fan issue or water pump ? It's the first time I've seen the warning light and it's only come on since I've topped the coolant up any ideas ... Ps I'm piss poor journey man mechanic so any help would be helpful

 

Thanks sounds promising is it next to the filler cap with a bleed 10mm bolt on top?

 

I've just purchased a Haynes manual from Amazon just now ... Sounds like a must have