As 1978 ground to its drear conclusion, Britain entered a particularly acute period of industrial strife, which came to be known as The Winter of Discontent. IPC, publishers of Cheeky Weekly, had already suffered the effects of industrial disputes earlier in the decade, and there were indications that trouble was again brewing when Cheeky Weeky dated 02 December 1978 was reduced from the usual 32 pages to 27. No reason for the page reduction was given in that issue, but most readers would have guessed that some sort of strike or work to rule was the cause. This thread on the Comics UK forum suggests that the majority of the disputes that affected IPC's output in the 70s were at the printers rather than at IPC itself.
The following week's issue was also truncated, although gaining an extra page on the previous week to make it only four pages shorter than normal.
Whatever the dispute that had caused the reduced issues, it must have worsened, as no Cheeky Weekly appeared for the following 3 weeks.
I don't remember where in London I was working at the time, but for a number of years I would visit newsagents near my various places of work during my lunch break on the day my comics of choice were published. I would then smuggle my purchases back into the office under my coat, as I didn't want my colleagues to know I was a comic reader. I don't recall asking the newsagent why there was no Cheeky Weekly during the period in question - I think I just assumed it hadn't appeared due to a strike or somesuch, but checked a couple of other newsagents, just to be sure.
We can be sure that no issues were published with cover dates 16, 23 and 30 December 1978 because the episode of Mystery Boy featured in the first issue after the break (dated 06 January 1979) continues directly from that in the 09 December 1978 issue in which Sandy was abducted by two German airmen. Also, there was a message across the top of page 2 of the 06 January 1979 issue (which was a full 32 pages) reading "Sorry we've been away - glad to be back for the new year, pals!"
It was particularly frustrating that one of the lost comics was what would have been the Christmas 1978 issue. However, I believe that most if not all of the material prepared for that issue was eventually used in one form or another - see these posts.