Lucky 7 Jackpot
I was looking for some 1K ZX81 listings and found a slot machine game by Artic Computing. The original listing is here.
I slightly amended the logic as attached (number range is 1 - 7 inclusive, winnings is 7 or 14 pence). You win if the reels add up to 7 (7 pence) or all reels match (14 pence).