The 'official' Dan Air website has mention of a Comet 4B (not 4C) G-APMB making an emergency landing at Gatwick (rather than Heathow) on a flight from Manchester to Milan on 25th March 1978 following a double engine fire warning. A few differing details, but quite a lot of matching. Is this the flight?
http://www.danair.org/
Click 'Letters' button on the left then the 'Bryan Wyat' link & scroll down a bit.
There's also a paragraph about this incident for the Comet 4B G-APMB in Martin Painter's excellent Comet book 'The DH106 Comet - an Illustrated History'
G-APMB also has special meaning for me as it was the last Comet I ever flew on way back in 1977 when I was a kid. This Comet survived at Gatwick until about a year ago.