(* I'm reposting this because of an interesting response I got from Richard Sheaf which pointed me to another Hampson-drawn advert (which I'm including a scan of below). First, the original post, then onto Richard's comments...)
The fact that Frank Hampson drew two episodes of 'The Adventures of the Bovril Brigade' is widely known amongst Hampson fans. The two strips are covered (amongst others) at The Lost Characters of Frank Hampson website which says that the Bovril half-pages ran in 1963-64. Well, I can pre-date them to 1962 when the ads appeared in Children's Newspaper.
The first ad. I've located (above) appeared 29 September 1962 (repeated 20 October 1962, 26 January 1963, 9 March 1963). The second ad. (below) first appeared 10 November 1962 (repeated 16 February 1963).
These adverts were repeated in various comics and were still appearing at least as late as December 1964 (in TV Comic).
Richard Sheaf comments: "I thought I'd drop you a line to say that the 'rain' strip first appeared in Eagle 17/11/62. It was then repeated in Eagle on 23/03/63.
"The 'horses' strip appeared just once (to my knowledge) on 12/01/63.
"The TV Comic 14/12/64 that the LC of FH refers to is, I believe, a different ad entirely, in that instance for Lego. Have you seen that ad? Have a look in Look and Learn 19/12/64."
So I did, and Richard is quite correct. The 1964 advert (which also appeared in Eagle, I believe) was 'Come to Legoland'.
Looking through my little list of Hampson's work, I see I have a note that he drew 25 episodes of a series featuring the Chalmers family for the National Coal Board which appeared in 1963-64. You can see some excellent examples at the Lost Characters of Frank Hampson website.